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Skywatcher F Stop (1 Viewer)

J

johnno

Well-known member
Hi all,

I need some advice,I have a Nikon D40 attached to a Skywatcher ED80 for Bird Photos.

Why are the Pics from the ED80 much brighter,almost overexposed,compared to Pics from a Tamron 70-300mm,used at the full telephoto length of 300mm.

The F stop on the Tamron is 5.6 @ 300mm,which should be brighter than the Skywatcher 7.5,@600mm

Yet, the Skywatcher can easily achieve shutter speeds of 1/1000 sec,and still give good bright Pics,while the Tamrons pics are dark at the same shutter speed.

I have just started using my ED80 with the Camera,so still learning.
I realise that increasing the shutter speed,when using the ED80 will decrease the over brightness,but cant understand why so much difference,when one lens is 5.6,and the other is 7.5...........I would expect the slower lens to be Darker,Not lighter.

I am Not Complaining,In fact Very Pleased,just confused

Regards.
John
 

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Well, the EXIF says the exposure on the Tamron was 1/1000 sec on aperture priority and the exposure on the Skywatcher was 1/320 sec in manual mode. Since the Skywatcher is less than one stop slower than the Tamron, an exposure of 1/640 would probably have been more appropriate.
 
Also, when you look at the aperture on the Tamron, it was f7.1.

Next to the 80ED's f7.5, there's vitually no difference.

But the shutter speeds are worlds apart - so had you brought SS of the 80ED shot down, or the SS of Tamron shot up, you'd have got more parity of shot between the 2 lenses

You say that the Tamron is f5.6, but thats just its max ap at 300mm, but the shot was presumably taken in Auto mode, as the data says f7.1 , not f5.6
 
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Like the others said shutter speed is the variable there. Try shooting both in manual mode, put the tamron at 300mm, F5.6 and whatever SS it needs for the image.
Then try the ED80 at the same speed and you'll have a better comparison base.
The ED80 will be darker than the Tamron but not as much as it sounds, maybe 1/3 of a stop.

I’ve made a few comparisons with my Nikkor 300 F4 and the ED80 was as bright as the 300mm at F/6.3 at infinity, slightly more...maybe F/6 but I couldn't check that on the camera. From my tests the ED80 (and pretty much any scope) have about half a stop or more light transmission than a photo lens, for the same aperture. That’s not a scope virtue though, it’s more a photo lens problem, all the elements they have inside cost a lot of light transmission, that's why in the past lenses used to have a F/stop and a T/stop (transmission).
 
Aegea said:
Well, the EXIF says the exposure on the Tamron was 1/1000 sec on aperture priority and the exposure on the Skywatcher was 1/320 sec in manual mode. Since the Skywatcher is less than one stop slower than the Tamron, an exposure of 1/640 would probably have been more appropriate.
Click to expand...

How do you check the EXIF on this picture? Sorry for my lack of knowledge.;-) Welcome to Birdforum!!
Regards,Steve
 
Thanks everyone for your very helpful advice.

It was quite late at night here,and I uploaded the wrong pic for the telescope.

Heres one taken at 1/1000 sec,using the telescope,and compared to the Tamron it is still way brighter.

Now,I understand the difference shutter speed makes to overall brightness.

Thanks for your help.

P.S forgot to mention this Power pole is a fair way from me,in the next street across,behind our house,the pics have not been retouched in any way,except to resize for posting.

Regards.
John
 

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Steve - There is an extension for Firefox called "EXIF Viewer". When this is installed, right clicking on the picture will bring up a menu which includes "View Image EXIF Data". Other browsers probably have something similar, but I only know about Firefox.
 
johnno said:
Heres one taken at 1/1000 sec,using the telescope,and compared to the Tamron it is still way brighter.

Regards.
John
Click to expand...

I just checked the EXIF

You've changed the ISO to 400 from the last 2 pics which were ISO 200, hence still bright

If you want to do a comparison, keep the settings as near the same for both if possible.
 
Like Fernando said, shoot one with the Tamron at 300 but set the aperture to 7.5. Then shoot another one with the SW80ED with the same exif in full manual. Where your metering sensor points will throw everything out too if second shot are not taken in full manual.

The two Exif viewer tips are excellent. Never knew they exist in the browser. I just downloaded the Exif Viewer by hpbw2k, voila...everything exposed. Thanks.
 
Thanks Aegea, I will have to check this out tomorrow. I was using Firefox and my boy has me using Google Chrome and I find Chrome much faster downloading pictures. Like night and day for me.
Regards,Steve

Aegea said:
Steve - There is an extension for Firefox called "EXIF Viewer". When this is installed, right clicking on the picture will bring up a menu which includes "View Image EXIF Data". Other browsers probably have something similar, but I only know about Firefox.
Click to expand...
 
Thanks Xs for the information. I replied to Aegea as well. I always wondered how people found this information on the pictures. I like Google Chrome for the fast picture downloads.
Regards,Steve

XsCode said:
Chrome has several extensions, I find Exif Viewer the best
Click to expand...
 
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