Exposure 101! (1 Viewer)

This follows on from Salty's excellent camera dilema thread, specifically for some of the recent questions. It aims at those not familiar or 100% comfortable with ‘camera technology’.


A photograph (assume digital for now) requires reflected light from the subject to hit the recording surface (e.g. the CCD sensor in a DSLR body) through the lens of your camera. This light will therefore produce the image that is eventually displayed on your computer screen. The amount of light that hits the recording surface will determine the amount of contrast, colour depth and detail of the recorded image.

The amount of light that can potentially travel to the camera is controlled by the aperture of the lens (i.e. the f number). The smaller the number, the larger the hole and therefore more light that can get to the camera. So ‘wide open’ means the smallest f number a lens can give, ‘one stop down’ is the next largest number (e.g. on a 400mm f5.6, wide open is f5.6. 1 stop down is f8). Note that a larger aperture (i.e. a smaller f number) will decrease the depth of field (DOF). So f4 will have less of the image in focus compared to f5.6 or f8.

Next in line is the camera shutter that opens, when you press the button, to expose the recording surface to light. So this also controls the amount of light that hits the recording surface to create the final image. The faster the shutter (bigger number displayed on the camera), the less light, but the more chance of ‘freezing’ motion (e.g. a flying bird) because the shutter is only open long enough to ‘see’, for example, a wing in a single position.

ISO – this was normally referred to as ASA with film and indicates the speed a film could capture the light presented to it. In the terms of digital, it is the speed that the recording surface (i.e. sensor) will capture the light. This is normally set by one stop increments, i.e. 200 ISO will capture light twice as fast as 100 ISO. However, faster (i.e. bigger numbers) will increase the amount of ‘noise’ in the photograph.

So, what does this mean to you as a DSLR user?

To correctly expose a photograph (i.e. get a bright image, without losing detail in the paler or darker areas and keep good colour rendition and contrast) you need to set the correct aperture, shutter speed and ISO.

How do you do this? First thing is to remember is that aperture, shutter and ISO are normally referred to in ‘one stop increments’ (aperture - f5.6, f8, f11 etc, shutter - 1/120, 1/250, 1/500 etc and ISO - 100, 200, 400 etc) so adjusting one of these should mean a one stop adjustment to another. For example, say the correct exposure should be f16, 1/120, ISO 100. You could adjust the ISO to 200 (so increase ‘1 stop’ faster light capture on the sensor) and the shutter to 1/250 (less light by ‘1 stop’) and still expose the image correctly, but reduce potential of blur from movement.

How do you know the correct exposure? All recent DSLRs have built in light meters that can automatically decide the correct exposure for your picture. The way the camera does this is based upon which setting you use.
In total Automatic, the camera will try to get the AVERAGE settings for depth of field and freezing motion based on the ISO setting. This will normally give pictures with more depth in focus, but loose ‘sharpness’ due to motion blur in moving subjects (such as birds).
To maximise the shutter speed, and therefore the ability to ‘freeze’ the frame, many nature photographers will normally use aperture priority. This usually means that the wheel close to the shutter button controls the aperture set on the lens (assuming the lens is automatic). The camera will then calculate the correct shutter speed based on reflected light, the set aperture and the ISO setting. By opening the aperture (setting a smaller f number) more light can reach the recording surface, so the shutter does not need to be open for as long. Therefore, the smaller the aperture, the faster the shutter speed required to get the same amount of light. This is similar to increasing the ISO, as described above.

The camera will also have other settings that controls the way that it ‘guesses’ exposure for the current view in a viewfinder (e.g. centre weighted, spot, etc), but I guess that is for the next instalment!

Finally, to answer another question, the camera's viewfinder will generally see the 'wide open' setting on the lens. Therefore an f2.8 lens will appear brighter than an f5.6 even though both may be set to f8...unless you press the DOF preview button on your camera!

Please feel free to correct my mistakes or assumptions. There is plenty more to say on automatic metering modes, use of aperture vs shutter priority, TCs, post-production etc.

mw_aurora said:

To maximise the shutter speed, and therefore the ability to ‘freeze’ the frame, many nature photographers will use aperture priority.

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Don't you mean shutter priority? If you want to freeze the action, isn't it better to use shutter priority, where you pick the shutter speed - you'd pick a fast one - and let the camera adjust the aperture automatically. If you use aperture priority, the camera might be using a slow shutter speed, which is what you are trying to avoid.

Otherwise, very nice summary!

Good post mw_aurora,

I like to keep things simple. I don't even bother metering. Instead, I tend to use manual mode, and make a best guess exposure(usually of a tree or some other inanimate object that is in the same kind of lighting that I expect to find my target) as soon as the camera is setup. Then I just review the histogram, which shows me the distribution of properly exposed pixels in the image, adjust my shutter speed as necessary, and repeat If the histogram does not look right. In practice, this only takes a few seconds, and I'm ready to shoot.

Now here comes the crucial bit, if you want to get the best out of your DSLR and utilize it's entire(hidden)dynamic range...

Firstly - Shoot in RAW mode (jpeg records less dynamic range, and is more lightly to blow out highlights and loose detail in the shadows!).

Secondly - "EXPOSE RIGHT" (which means, look at your HISTOGRAM and make sure the largest proportion possible of pixels is skewed over to the right side of the histogram, but not too far right or your highlights will start to clip/get blown out!).

Here's an article which explains the "expose right" technique:
http://www.luminous-landscape.com/tutorials/expose-right.shtml

Here's another, which goes into much greater depth about RAW converters and post-processing to maximize dynamic range:
http://www.normankoren.com/digital_tonality.html

And, last but not least, this article which has the best technique I have yet come across using Photoshop layer masking to bring out all that hidden dynamic range:
http://www.phong.com/tutorials/mask.tree/

I have yet to try the above layer masking technique, but it seems much better than most of the other ways which I have tried and seen recommended!

Leo

RAH said:

Don't you mean shutter priority? If you want to freeze the action, isn't it better to use shutter priority, where you pick the shutter speed - you'd pick a fast one - and let the camera adjust the aperture automatically. If you use aperture priority, the camera might be using a slow shutter speed, which is what you are trying to avoid.

Otherwise, very nice summary!

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Actually, I'm with Mark on this. I tend to use aperture priority and set the aperture as wide as I'm prepared to go with that particular lens (usually half a stop down but occasionally wide open) knowing that I've then got the fastest shutter speed possible. If that still isn't fast enough I'll adjust the ISO upwards. The reason I do this is that I'm almost always trying to isolate the subject from the background and if you go to shutter priority you're giving away control over the DOF. I guess that, as this thread is intended as a primer, I should explain that the wider open the aperture (the lower the f number) the narrower the depth of field so focus is more critical with a wider aperture but it helps when trying to achieve that smooth out of focus background.

This thread is a great idea Mark. I'm wondering if it might not be useful to have it 'stickied' at the top of the camera forum?

Sean

Yes, Aperture-priority is best - if you choose shutter-priority and the light changes you could easily run out of apertures and under-expose.

It's worth stating that with long telephotos changing the aperture by a stop either way isn't going to make much difference to 'depth-of-field' (the amount of the image that is acceptably sharp either side of the point of focus).

Aperture priority - which can be easily flicked either way to retain enough shutter speed. I try to always flick back to P mode between taking pictures as a safety option - I've been caught out trying to capture a sudden opportunity to photograph a flying bird with no time to change the settings. That way i can usually rely on getting something. High ISO may mean more noise but noise is easier to treat afterwards than camera shake.

Aperture priority(again) - its worth searching out the DoF tables for your lens, tables which will show the maximum and minimum distance a subject will remain sharp for a given focal length and aperture.

Ray

Thanks everyone for the positive comments. Rather than loads of individual replies...

RAH, I did mean Aperture priority, although having read the paragraph again, I will update it to be more explicit! Personal preference is important here though. Now updated the original post.

Leo, good links and agree with the principles. I think some explination of other methods of judging correct exposure (e.g. in-camera metering) would help with the understanding.

Sean, agree with your technique 100%. I'm happy for this to be a sticky and to update my post(s) where/when required.

Adey, agreed, although doesn't this depend on the distance from the camera to the subject (i.e. larger difference to a close subject)?

greypoint, good point about trying to ALWAYS return the camera settings to something you know after a series of shots. This means that you know exactly what the camera settings are and can change them without thinking.

Ray, good point. Fancy explaining how to read them in laymans terms?

One thing I didn't mention - I am not an expert and do not intend to prescribe a best way of doing things. My intentions are purely to explain some basics and let people decide their own favoured methods.
Following on, I have some thoughts on in-camera metering, which I will try to add to the thread soon (unless someone else beats me to it!).

Cheers Mark.

Mark,many thanks,I am going to print this page,and keep it for referral.I always thought that if pics come out somewhat overexposed,one just slides the exp bar down a little,so that one is on the minus side ,and not the plus side.

christineredgate said:

Mark,many thanks,I am going to print this page,and keep it for referral.I always thought that if pics come out somewhat overexposed,one just slides the exp bar down a little,so that one is on the minus side ,and not the plus side.

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Thanks Christine (and for the kind words on Salty's thread). I am afraid I updated the original post, probably just after you printed it...sorry!

Generally you are spot on for the exposure bar.
However, I think you have seen and as said by SeanKP, usage really depends on:
the lighting on the primary subject (for me, normally the blurred bird whizzing past) compared to that on the background (normally the blurred mess as I try to keep the blurred bird in frame) and, if it wasn't hard enough already, the metering mode on the camera...
Why do we do this? |:d|

Cheers Mark.

mw_aurora said:

Adey, agreed, although doesn't this depend on the distance from the camera to the subject (i.e. larger difference to a close subject)?

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Yes, but we're all going to use fieldcraft to get as close to the subject as we can to avoid all those problems of haze, trying to enlarge a tiny section, etc., etc., aren't we... :eek!:

Some (interesting?) facts on apertures! :h?:

F stops obviously worry a lot of people :stuck: so what are they?

Well, from a bird-photographers' point of view high shutter speeds are probably the most important consideration so they're usually left with whatever aperture suits the particular shutter speed chosen. From a practical point of view, then, you don't need to worry too much about them!

Shutter speeds are easy to understand as you're only dealing with one dimension - time - and doubling or halving the shutter speed makes one 'stop' difference up or down, whereas with apertures there are two dimensions as it's all about area - and a (roughly) circular area at that.

This is why a one stop 'slower' aperture is only (approx.) 1.4 times the previous number rather than 2 times (F1 - F1.4 - F2 - F2.8 - F4 - F5.6, etc.) and that F number is actually a ratio.

The ratio is focal length : aperture and a lens with a maximum of, say, F4 is described as F1:4. Divide the second number into the first and you have the actual aperture size which is what determines the depth-of-field. The smaller the size of the aperture, the greater the depth-of-field.

This is, fundamentally, why a telephoto has less depth-of-field than a shorter focal-length lens: Divide a 400mm lens by 4 and you see that you need an aperture of 100mm whereas a 40mm lens would have a 10mm aperture for its F4 setting. To get that 10mm opening on your 400mm would require an aperture of F40 and a very slow shutter speed to cope with the vastly reduced amount of light.

Depth-of-field, the amount of the image that is recorded as acceptably sharp on the film/sensor, is all to do with the angle that the light rays from the rear of the lens strike the film/sensor - please tell me you don't want a diagram to explain! - oh, and the distance you are from the subject alters the depth-of-field as well. The closer you are, the less you get - if you look at the depth-of-field table that you get with your lens you'll see how dramatically different it can be from a distant setting with the lens stopped-down several stops to a close-up at full aperture. Some macro lenses, apparently, have no d-o-f at all at the closest distance at full aperture!

Adey Baker said:

Well, from a bird-photographers' point of view high shutter speeds are probably the most important consideration so they're usually left with whatever aperture suits the particular shutter speed chosen. From a practical point of view, then, you don't need to worry too much about them!

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Don't mean to be argumentative, but you just described shutter priority mode. And this way of thinking is exactly why I said earlier that shutter priority is the way to go for beginners. Yes, of course one can use aperture priority and watch the shutter speeds that are obtained, and do it this round-about way. But the easiest way to think about it, especially for beginners (whom this posting is supposed to help) is shutter priority.

I agree completely that using aperture priority and setting the aperture at full is a great way to get the fastest shutter speed possible for your camera while still getting a proper exposure. However, for beginners, I think that shutter priority is much more straight-forward and easier to understand, as your above post illustrates.

RAH said:

Don't mean to be argumentative, but you just described shutter priority mode. And this way of thinking is exactly why I said earlier that shutter priority is the way to go for beginners. Yes, of course one can use aperture priority and watch the shutter speeds that are obtained, and do it this round-about way. But the easiest way to think about it, especially for beginners (whom this posting is supposed to help) is shutter priority.

I agree completely that using aperture priority and setting the aperture at full is a great way to get the fastest shutter speed possible for your camera while still getting a proper exposure. However, for beginners, I think that shutter priority is much more straight-forward and easier to understand, as your above post illustrates.

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We could debate this for ever - I think a beginner would perhaps not keep a check on everything going on in the viewfinder so could easily miss the fact that their chosen shutter speed may not provide enough light for full aperture whenever light levels drop thus causing under-exposure.

So,is the max aperture for a 300f4 lens,f4,or can it be altered /moved down /up/to F2.If I am using the 100-400 lens at its max setting,that would be 400,would I be able to alter the F no.Also whilst here,may I ask,if using the 100-400 with the 2x it has been mentioned that due to the loss of light stops,one should alter the F nos.So which setting should be used on the cam20d/350d,and which f,stop should be used,also the same with the 300F4 using the 1x4 .I ask this,as I find the larger lenses too heavy/too expensive.But using the extenders on the said lenses does increase the magnification.Fortunately the 300 uses auto focus instantly with the 1x4 but I find I have to use manual focus with the 2x.But I did manage a half decent shot with the 100-400 plus 2x,it was dark,but once highlighted was acceptable,but I could have perhaps improved the shot,if I had used some different setting.Just used P mode.

Adey Baker said:

Depth-of-field, the amount of the image that is recorded as acceptably sharp on the film/sensor, is all to do with the angle that the light rays from the rear of the lens strike the film/sensor - please tell me you don't want a diagram to explain! - oh, and the distance you are from the subject alters the depth-of-field as well. The closer you are, the less you get - if you look at the depth-of-field table that you get with your lens you'll see how dramatically different it can be from a distant setting with the lens stopped-down several stops to a close-up at full aperture. Some macro lenses, apparently, have no d-o-f at all at the closest distance at full aperture!

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Can we have diagrams please Nice explination, and you knew what I meant with my original reply - thats what extension tubes are for isn't it? :stuck:

Christine, your 300mm's maximum aperture is F4 - you can't improve on it in any way.

Divide the 4 into the 300 and you get 75mm which is the maximum opening you can get on that lens (that's why your filter thread is 77mm).

If you put, say, a 2x converter on then you've made it a 600mm lens but by doing the above sum the other way round you can see why it's now F8: i.e. divide the 75 into the 600 and you get 8.

You shouldn't have to alter any settings as (I think) the camera will automatically 'see' the lens as having a maximum aperture of F8 and will give you the correct shutter speed to match - assuming you're using one of the auto-exposure modes.

mw_aurora said:

Can we have diagrams please

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I'm not sure what my skills are like in Photoshop, whether I can concoct an easy to understand diagram - don't hold your breath!

Thanks,Adey,I feel much happier now knowing that the cam will do everything for me.I take it that the P setting is one of the auto exposure modes.