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Loss of light when comparing lenses at 135mm


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#1 Debbus

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Posted 24 November 2016 - 09:21 AM

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Hi guys,

Very new here, and very new to the photography world. But it's surely becoming more than a hobby to me. I shoot with the A7S2 and I just bought my first manual vintage lens. It's a bit of a weird one, and pretty rare, but I've done my research and I was expecting great results. The lens is a Vivitar 135mm f2.8 Close Focussing which goes up to a 1:2 magnification. It comes in a Minolta MD mount and I've adapted it to the E-mount with a dumb adapter. I tried it out and I'm impressed. It handles beautifully and it's super sharp. The only thing I noticed while using it is that I was expecting it to let in more light than it actually does. So I did a little experiment. I set up my Sony FE 24-240 F3.5-6.3 on the same tripod, same lighting conditions, I set it to approximately 135mm and shot to expose properly at f6.3 and f11. After that I slapped on the Vivitar, same shutter speed, same ISO, same aperture values, and guess what... the Vivitar was not only much darker but also much narrower.

Time for examples, straight out of the camera, I only combined the two in one image:
https://goo.gl/photo...idstqXRiABowj4A

Here you can see the Vivitar on the left, the Sony on the right, this is at f6.3. I also have one where I repositioned the tripod so the angle of the Sony would resemble the Vivitar, and this time at f11, the other way around with the Sony left and Vivitar right:

https://goo.gl/photo...sgoW2coJXhNeLp9

So I know it's an old lens, and the difference in angle probably had something to do with the Close Focussing feature. But the loss of light I can't explain, and it's no small difference. Light entering the camera through different lenses with same apertures should be the same right? Any thoughts on what may be the cause of this?

#2 timde

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Posted 24 November 2016 - 11:29 PM

The transmittance of the glass (the T-stop) and vignetting are likely the cause. They are normal factors.



#3 Golem

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Posted 26 November 2016 - 09:20 PM

Apparently your Sony lens is somewhat less than honest,

numerically speaking. The 135/2.8, despite being a Vivitar,

is very likely exactly what it claims to be cuz a 135/2.8 is a

simple design and easy to correctly manufacture.

 

Your Sony lens loses speed as it increases FL and since it

has reported a "dishonest" FL ... looking more like 90mm  

than 135mm ... it's prolly also reporting the f/stop value its 

firmware assigns to its 135mm configuration but is actually 

exposing at the [faster] f/stop associated with 90mm.  

 

Instead of setting the Sony lens to "approximately 135mm" 

set it to the same image size on your preview screen. Use 

a subject distance of at least several feet.  Set both lenses

to an indicated f/8. The result should show a rather similar 

brightness between the two lenses. 

  

You provided close focused examples. The Vivitar image 

would have been brighter if you had left the image blurry 

and the lens focused to 10 ft [+/-]. To sharpen the image at 

close range you extended the focus mount, and that cost 

you some effective lens speed. The Sony lens does not 

extend its barrel to focus closer. It shortens FL to effect a 

closer focus. This does not cost any brightness but it does 

result in the smaller image. 

 

One final and very important thing: If you do these shots by 

ordinary household illumination, and if that illumination is 

LED lights, keep the shutter speeds below 1/30 sec. If you 

use a higher speed, the lighting itself can cause far more 

exposure difference than shown in your sample shots, and 

the effect is random. This effect may or may may not be in 

play in your sample shots ... unless they're at 1/15 sec :-) 


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#4 Debbus

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Posted 26 November 2016 - 10:13 PM

Perfect explanation, thanks! I will try a few more with the camera at farther distance. I found out looking at EXIF that the angle of the Sony was actually at 124mm.

These shots were made with very little light and long exposures, 3 sec or more, to keep ISO at 100.


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