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07-02-2013, 05:49 PM   #1
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Same settings, different lens, totally different results?

Maybe my understanding is wrong, if so, please correct it...
To me, it makes sense that if I use the same camera settings (in this case, iso 200, f/8, 1/100, exp comp -1, same metering mode), I should get the same results.

For the first photo, I used the Tamron 70-200 2.8 ( iso 200, f/8, 1/100, exp comp -1, @ 140mm) with continuous lighting.




I thought to myself "ok, looks good, but maybe using a true 1:1 macro lens will give me nicer results", so I swapped lenses.

This time, I used the Tamron 90 2.8 ( iso 200, f/8, 1/100, exp comp -1, @ 90mm) with the same continuous lighting, positioned the same way. The results look totally different from the first.




So, now tell me, should this be happening; should the results be so different?

And if it isn't the camera settings or lighting conditions making the difference, what is? The focal length? Or is it something to do with the fact that the 70-200 has a 77mm filter opening whereas the 90 has a 55mm?

07-02-2013, 05:55 PM - 1 Like   #2
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f2.8 is physically different on the two lenses in real diameter of the aperture opening. That should result in more light being admitted via the physically larger 77mm opening on the 70-200 than on the 55mm opening on the 90.
07-02-2013, 06:11 PM - 1 Like   #3
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Possibly pupiliary magnification factor (p)

I think there is some confusion about the exposure setting--if you set the shutter speed, iso, fstop, then the exposure mode is manual--if you set it to -1 ev--then the camera is possibly resetting something.

If you set all the same then it is M (manual) and the magnification is the same.

If the exposure settings are truly the same (and the lenses are not terribly wrong on f/)--then the only difference is the lens cosnstruction. The difference being the pupiliary magnification factor (p). Assuming p~0.5 vs 1.0 for a true macro, I should think an exposure difference of 2 stops is likely--I did not look up the equations/calculate it.
07-02-2013, 06:15 PM - 1 Like   #4
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By the way I assumed you were at a maginification (m) of about 1. At normal distances (m < 0.1) the difference in lens construction (p) is likely insignificant .

07-02-2013, 06:15 PM   #5
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QuoteQuote:
f2.8 is physically different on the two lenses in real diameter of the aperture opening. That should result in more light being admitted via the physically larger 77mm opening on the 70-200 than on the 55mm opening on the 90.
f-stop is f-stop, the actual diameter makes no difference.
07-02-2013, 06:22 PM   #6
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If the magnification is small (so p is unimportant) and the light sources are not point sources, then the same f stop will result in the same exposure light evel. The iris diameter divided by focal length determines f stop and thus exposure.
07-02-2013, 06:29 PM   #7
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Obviously either the 90mm lens has some serious light transmission losses or its aperture is not correctly calibrated..

Tis could be due to a poor electrical contact which causes the camera to be reading the incorrect aperture when metering or something in the lens mechaincally
07-02-2013, 06:31 PM   #8
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Presumably the zoom has a p value much less than 1, and the macro lens is close to 1--if so the zoom should be underexposed. Did you mix up the captions? If not seems likely the exposure settings were not the same.

07-02-2013, 06:35 PM   #9
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Actual light gathering ability isn't really meassured in F-stops, it's called T-stops. Though the differance looks huge.
07-02-2013, 06:42 PM   #10
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With multi-coating normal (non-mirror) lenses f stop and t stop are virtually the same.
07-02-2013, 06:51 PM - 1 Like   #11
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Check both lenses to see if the aperture blades are opening and closing freely.

How strong was the continuous lighting? To get a good exposure at ISO 200 f/8 and 1/100 shutter, you need a lot of light, fairly close to diffused daylight.

It may well be that the dark image was exposed at f/8 while the bright one at f/2.8 (or so) if the aperture blades were not fast enough to close down to f/8 on the 70-200 lens. DOF is very shallow on the bright image further suggesting a wider aperture.

You should never expect the same exposure from two different lenses even when using the same exposure settings (ISO, shutter speed and f-stop). The f-stop is the ratio of the focal length over the diameter of the iris. It does not account for the effective transmission through the glass elements. If you want to have identical exposures then you should use lenses with T-stop markings as in video.

Having said that, the differences between the 2 photos is so dramatic that unless something else went wrong, then your 70-200 most likely has sticky aperture blades.
07-02-2013, 06:58 PM - 1 Like   #12
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Did you take a couple of pics with each lens? What kind of continous lighting did you use? Many light sources exhibits heavy flicker.
QuoteOriginally posted by dms Quote
With multi-coating normal (non-mirror) lenses f stop and t stop are virtually the same.
Yep, that is why I wrote "but the difference is huge" concerning the pics. It can explain some variation, but not one this big.
07-02-2013, 07:46 PM   #13
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Thank you very much to everyone that has replied thus far!

QuoteOriginally posted by dms Quote
Presumably the zoom has a p value much less than 1, and the macro lens is close to 1--if so the zoom should be underexposed. Did you mix up the captions? If not seems likely the exposure settings were not the same.
Nope, captions are correct and settings were same. If you go to the Flickr link and look up at the EXIF data on both photos, they match up.

QuoteOriginally posted by demp10 Quote
Check both lenses to see if the aperture blades are opening and closing freely.

How strong was the continuous lighting? To get a good exposure at ISO 200 f/8 and 1/100 shutter, you need a lot of light, fairly close to diffused daylight.

It may well be that the dark image was exposed at f/8 while the bright one at f/2.8 (or so) if the aperture blades were not fast enough to close down to f/8 on the 70-200 lens. DOF is very shallow on the bright image further suggesting a wider aperture.

You should never expect the same exposure from two different lenses even when using the same exposure settings (ISO, shutter speed and f-stop). The f-stop is the ratio of the focal length over the diameter of the iris. It does not account for the effective transmission through the glass elements. If you want to have identical exposures then you should use lenses with T-stop markings as in video.

Having said that, the differences between the 2 photos is so dramatic that unless something else went wrong, then your 70-200 most likely has sticky aperture blades.
It is kind of beginning to look like that... I need to do some more comparisons and report back.

QuoteOriginally posted by VisualDarkness Quote
Did you take a couple of pics with each lens? What kind of continous lighting did you use? Many light sources exhibits heavy flicker.

Yep, that is why I wrote "but the difference is huge" concerning the pics. It can explain some variation, but not one this big.
Yes, I took multiple photos with both lenses at the same settings, they turned out the same as the above two I posted
For lighting, some old Smith Victor reflector lights with some flourescent, 23Watts 1210Lumens, 4100K Color Temp bulbs.

This is what the set-up looked like (except in the two photos I posted, it was still sort of light outside, so I only used the one light. I'm trying some more tests now and using the second light.)

07-02-2013, 08:02 PM   #14
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QuoteOriginally posted by demp10 Quote
DOF is very shallow on the bright image
I think Demp nailed it here. The darker photo is completely in focus, while the lighter one has the classic 'wide open' shallow DOF, only the front is in sharp focus. Also notice the glass jar. First photo, completely blurred out. 2nd photo, it's nearly in sharp focus.
Without knowing the data, I'd guess the top photo was shot at f/2.8 to (maybe) f/4.0. The second one? Much greater DOF, lens stopped down.
Also, unless the photos are cropped to the same image size, there's a difference in the camera-to-subject distance. The zoom shot was at 140mm FL, 50% (roughly) longer than the 90mm macro. This may not account for the exposure or DOF differences, just thought I'd point it out.
Ron

Last edited by rbefly; 07-02-2013 at 08:15 PM.
07-02-2013, 08:09 PM   #15
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QuoteOriginally posted by demp10 Quote
Check both lenses to see if the aperture blades are opening and closing freely.

How strong was the continuous lighting? To get a good exposure at ISO 200 f/8 and 1/100 shutter, you need a lot of light, fairly close to diffused daylight.

It may well be that the dark image was exposed at f/8 while the bright one at f/2.8 (or so) if the aperture blades were not fast enough to close down to f/8 on the 70-200 lens. DOF is very shallow on the bright image further suggesting a wider aperture.

You should never expect the same exposure from two different lenses even when using the same exposure settings (ISO, shutter speed and f-stop). The f-stop is the ratio of the focal length over the diameter of the iris. It does not account for the effective transmission through the glass elements. If you want to have identical exposures then you should use lenses with T-stop markings as in video.

Having said that, the differences between the 2 photos is so dramatic that unless something else went wrong, then your 70-200 most likely has sticky aperture blades.
Ok, went ahead and checked all my lenses for snappy aperture (like the guy in the video below) and everything seems fine on all lenses including the 90 and 70-200. I just went ahead and cleaned the contacts on the camera and lenses to make sure that wasn't the issue.

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