how to compute 35mm equivalent for digiscoping (1 Viewer)

Hi

One of my DSLR friend asked me the question:

Since I am using the Nikon P300 with the Nikon 82mm Fieldscope, what is the 35mm equivalent if the camera focal length is 6.3mm?

Is there any formula to compute the 35mm equivalent from the DC's focal length?

Cheers

See Toh

Try the digiscoping calculator on the link below. You should be able to manually input all your known values to come up with a 35mm equivalent.

http://www.jayandwanda.com/digiscope/digiscope_calc.html

If you take a photo at 6.3mm does the exif data in the photo tell you the 35mm equivalent. You could then work back and find the focal length multiplier that the particular camera is using. I don't think there's one simple formula for working this out as digital camera lenses/sensors all vary so much. You would generally need to know the size of the sensor and compare how much smaller it is than 35mm film. If for example the sensor was 4X smaller and the camera was at 6.3mm then you would do 6.3 x 4 = 25.2mm equivalent in 35mm terms. Some cameras work out the figure using the diagonal comparison to 35mm and some use a horizontal comparison which adds further confusion.

Paul.

Hi Paul

Thanks for your reply.

I just found out that if you upload your pictures onto Flickr, it compute the focal length in 35mm format.

The focal length of 7.2 mm in a P300 for example is equivalent to 40 mm in 35mm terms. Multipy by the 30x eyepiece, it is equivalent to 1200 mm in 35mm terms when attached to the fieldscope.

Therefore, for my digiscoping setup using 30x DS eyepiece, the formulae I have to remember is just P300 focal length x 5.556 x 30.

Regards

See Toh

Toh, with the examples you are using I am not sure what you are trying to calculate. Are you are trying to describe to your friend how much magnification, focal length, your camera and scope will produce as compared to his SLR with say a 500mm lens.

If that is what you are trying to do, it is quit simple. Here are the specks for your camera.

24-100mm (equivalent) f/1.8-4.9 lens with optical stabilization

Notice it says (equivalent) That means the 35mm conversion has all ready been made. For the most part, you only need a multiplier with some SLRs. When you have your lens at the lowest setting, it is the equivalent of a 35mm with a 24mm wide angel lens . At the highest, 100mm, it is the equivalent to a 35mm with a 100mm lens. If you are somewhere near the middle, it would be approximately 24 + half of the difference between 24 and 100.

!00 - 24 = 76 76/ 2 = 38. 38 + 24 = 62

Any other amount of zoom can be calculated in the same manner.

Now for the scope and camera. If the camera is 24mm and the scope eyepiece is 20x, you multiply 24mm X 20 which = 480mm. 25X = 24 X 25 or 600mm. 30X = 24 x 30 or 720mm ETC.

The camera on 100mm and the scope on 20x = 2,000mm. 25x = 2,500 30x = 3,000 ETC.

The camera in the middle which we determined was approximately 62mm. 20x = 1,240mm. 25x = 1,550mm. 30x = 1,600

For all practical purposes, multiply the camera's mm, determined by how far it is zoomed, by how much you have the scope zoomed. These numbers far surpass anything you can achieve with an SLR for subjects that will stay in one place long enough to get a picture. That is why it is so popular for wildlife.

Let the distance to the subject determine how much magnification you use but keep this in mind, both the camera zoom lens and the scope zoom eyepiece lose light the more you zoom them. If you have lots of light, it is not as much of a concern as it is for me in the Pacific NW this time of year with low light levels combined with overcast skies. Your P300's aperture is 1.8 @24mm and 4.9mm @100. That is almost 2 shutter speeds slower under the same light conditions when it is at full zoom.

This time of year I keep our scope between 20x and 25x with the camera 3/4 to full zoom using aperture priority mode being mindful to keep it as open as possible depending how far I zoom. In the summer I go to 30x or a bit higher while watching the shutter speed to make sure it is fast enough with usable ISO.

Hope this is what you were asking about and is of help.

Paul Cornfield, the link you posted didn't work for me. I would be interested to see it. Bob

Thanks Bob for your long response which answered fully my question as my friends always ask me the focal length in 35mm terms I am digiscoping at so that they could compare with their DSLR setup. As I use the 30x fixed eyepiece, the focal length is up to 3000mm if I fully zoom on my P300. Most of them use Nikon 600mm. So even if they use a 2xTC, the maximum focal length they could obtain is 600 x 1.5 crop factor x 2 = 1800mm.

After playing with my digiscoping setup, I realized that if the subject is not too faraway, to obtain a similar size image, I get better image quality by zooming the camera just enough to eliminate vignetting and then crop more during processing as compared to bigger/full zoom on the camera with lesser crop.

The system aperture at full zoom fl=3000mm effective is F6.5. Most digicams start to show mild softening after 5.6 due to diffraction. Take a test pic with the camera (no scope) in Aperture Priority at each zoom step and let us know what the Exif data shows for lens focal length and F ratio and I'll plug the data into my spreadsheet for analysis and tell you where the sweet spot is.

The P300 has a total of 13 zoom steps as appended below. Only at 6.3mm zoom step will the vignetting be fully eliminated. Slight vignetting at 5.5mm.

4.3mm - f/1.8
5mm - f/2
5.5mm - f/2.1
6.3mm - f/2.5
7.2mm - f/2.6
8.1mm - f/2.8
9mm - f/3.3
9.9mm - f/3.5
11.1mm - f/3.7
12.5mm - f/4
13.8mm - f/4.2
15.2mm - f/4.5
17.9mm - f/4.9

Here ya go SeeToh.

THEORETICALLY the camera lens settings where vignetting is minimized is at the green bar and the blue bar are the values for sharpest pics.

Thanks RJM. So does it mean that shooting at 12.5mm f/4 with my setup would produce the most ideal effect?

To phrase my question in another way, is it better in terms of image quality to shoot a particular bird at fullframe at 8.1mm (f/2.8) by getting nearer to the subject or to shoot it at the same fullframe size but at 12.5mm (f/4) and without having to move that close to the bird? Or am I asking an idiot question here

It is always best to get as close as you can. THe closer you get the more degrees of freedom the camera's exposure routine has to work with.

The blue bar is the solution if you want to crop instead of using more mag like you indicated in the previous post. From this point AF performance and shutter speed rapidly gets worse as you increase eyepiece mag and A4-sized prints will begin to show diffraction artifacts.

RJM said:

It is always best to get as close as you can. THe closer you get the more degrees of freedom the camera's exposure routine has to work with.

The blue bar is the solution if you want to crop instead of using more mag like you indicated in the previous post. From this point AF performance and shutter speed rapidly gets worse as you increase eyepiece mag and A4-sized prints will begin to show diffraction artifacts.

Click to expand...

Understood the technicalities now. Thanks for the tip.

BobnBernie77 said:

Paul Cornfield, the link you posted didn't work for me. I would be interested to see it. Bob

Click to expand...

Corfield, not Cornfield but it's a common mistake. :-O From 7th century Olde English it means Corf (cutting) the field.

The link works fine for me but there have been odd times in the past the page doesn't show up for whatever reason. Keep trying at various times of the day and it should work.

Paul.

1) effective focal length (focal length of the camera lens - scope combination)

f = 4.3 mm (focal length of the camera lens, minimal focal length 4.3 mm instead of 6.3 mm)
m = 25-75× (magnification)

f' = f * m

m = 25 → f' = 4.3 * 25 = 107.5 mm
m = 75 → f' = 4.3 * 75 = 322.5 mm

2) crop factor

1/2.3" sensor:

diagonal = 7.70 mm
width = 6.16 mm
height = 4.62 mm
crop factor = 5.62
http://en.wikipedia.org/wiki/Image_sensor_format

d = √(6.16^2 + 4.62^2) = 7.70 mm (diagonal calculated)

35 mm film:

d0 = √(24^2 + 36^2) = 43.27 mm (diagonal)

Crop Factor

FLM = d0 / d = 43.27 / 7.70 = 5.62 (Crop Factor calculated)

3) focal length equivalent for 35 mm film

EFL = f' * FLM

m = 25 → EFL = 107.5 mm * 5.62 = 604.15 mm
m = 75 → EFL = 322.5 mm * 5.62 = 1812.45 mm

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You can also compute focal length equivalent for the camera lens alone:
4.3 mm * 5.62 = 24.166 mm
and later for the camera lens - scope combination:
m = 25 → EFL = 24.166 mm * 25 = 604.15 mm
m = 75 → EFL = 24.166 mm * 75 = 1812.45 mm

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You can perform calculations using this calculator:
JavaScript Calculator

This one can calculate relation between focal length, object distance, object size, sensor dimensions, image size and scope magnification:
http://darekk.com/kalkulator/

RJM said:

Here ya go SeeToh.

THEORETICALLY the camera lens settings where vignetting is minimized is at the green bar and the blue bar are the values for sharpest pics.

Click to expand...

Hi RJM - Another question. Does your chart work for the 50x eyepiece also?

SeeToh,

Here is a chart for THEORETICALLY optimized eyepiece magnifications that provide the max image resolution for each zoom step on the Nikon P300 when used with the Nikon ED82. Using more magnification is just "empty" as the camera cannot resolve any more detail. Any less, and the camera does not utilize the scope's full aperture/resolution.

Bottom line is if you need more image scale than this optimized magnification provides, it may be wiser to crop the image rather than increase mag or lens zoom which will only leads to slower shutters speeds and/or higher ISO/sensor noise. Of course, the crop/more mag/zoom decision rests on the final output.

While the theory is fun, in practice the "gotcha" is the fact you have limited eyepiece magnification choices. But at least the chart reveals why the 30x Wide DS achieves the greatest number of practical effective focal lengths while the 50x and 75x eyepieces are not so useful.

Paul Corfield said:

Corfield, not Cornfield but it's a common mistake. From 7th century Olde English it means Corf (cutting) the field.

Click to expand...

Paul,

The Oxford English Dictionary makes has no reference to corf as a verb, lists it as a noun meaning basket, dating from the 15th century.

aostling said:

Paul,

The Oxford English Dictionary makes has no reference to corf as a verb, lists it as a noun meaning basket, dating from the 15th century.

Click to expand...

Ah, found it. The Old English word is cyrf, which does indeed mean "cutting." From this comes our words carve, and kerf.