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Why do my Pentax primes expose 1/2 3/4 stop brighter then the Tamron 17-50?

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Bob Corson

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I wanted to see how my Tamron 17-50, Pentax DA 35 and F 50 1.7 compared from a sharpness perspective across the image. Set up a quick and dirty experiment where I took a picture of the same surface with each lens at f4 (now I know I should have used a tripod and got exactly the same distance and exactly the same image but I was short of time). What I saw was very similar results as far as my eye says except for two things that surprised me:

Both the primes exposed brighter then the Tamron and the F 50mm seemed to have a slightly smaller field of view. Knowing that I had not used a tripod I tried it a few times I assume that even though the F is full frame the two lenses should have the same field of view set at 50 mm on the APS-C sensor and therefore it must be I am holding one closer to subject but I did seem to consistently get about a slightly greater field of view on the Tammy. I guess I should retry tomorrow with the tripod.



Anyway, comments re the exposure?



Thanks



This from the Tamron
This from the Tamron



And from the Pentax F 50
And from the Pentax F 50



--
Bob Corson
To truly see one must open their mind as well as their eyes
 
The Tamron is known to be a particularly dark lens. It was the only strike I could find against it, and was ultimately the reason I sold mine...
 
JayBratcher said:
JayBratcher wrote:

The Tamron is known to be a particularly dark lens. It was the only strike I could find against it, and was ultimately the reason I sold mine...
Click to expand...
Ok then I think I will set +2/3ev when I use it and see how that does.

Thanks
 
The only reason I can think of is that the lens stops down more. So Tamron's idea of f4 is a bit smaller than Pentax's. Is it consistent for all apertures?
 
More elements, therefore more glass-air surfaces and also worse coating, there are F-stops and T-stops, the F-stop is the size of the aperture relative tot he focal length, the T-stop is actually how much light makes it through the lens. With every uncoated glass-air surface a lens loses 5% of the light, with modern coatings its usually a lot closer to 0.5% or even better. however the more surfaces the more light is lost.

Consider that amazing Olympus 14-35mm F2.0 lens, it has 34 glass-air surfaces which is a huge amount by any standard, because of that the T-stop is actually T2.77 It loses 1 stop of light even though it has modern lens coatings. Now that is an extreme example, what you're seeing isn't so extreme.
 
While aperture may be set the same, light transmission may be different between lenses, that's why in cinematography, they use T-stops and not F-stops.

FYI, the 31mm (tried two copies) at f1.8 is darker than the Sigma 30mm at f1.8.
 
I noticed the same thing with my Tamron 17-50 and an A28/2.8

At f/2.8, the exposures were about the same.

At smaller apertures, the A28 was noticeably brighter.

I thought something might be wrong with the used A28 but finally chalked it up to something like T-stop. I figured the exposures (wide open) matched because, maybe, the A28 isn't quite a 2.8 lens (I don't suppose the Tamron is faster than f/2.8)... Maybe the A28 is really f/3.2 or so and this is why it matches the Tamron. Although, you can see I found about a stop difference at both f/4 and f/5.6

Anyway, you are not alone. The exposure I got with the Tamron never seemed wrong to me. Not until I also decided to check sharpness versus the A28 and K24.

I found the 28 and 24mm lenses to be about as sharp as the Tamron. Or, vice versa. However you wish to compare which to what.

Exposure at f/2.8 is close enough for me. At f/4 and f/5.6, I have to decrease shutter a stop with the Tamron. Then histograms pretty much match.

Exposure at f/2.8 is close enough for me. At f/4 and f/5.6, I have to decrease shutter a stop with the Tamron. Then histograms pretty much match.

Interesting, the difference in shape of histogram between 17-50 and A28, yeah?
 
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torrent999 said:
torrent999 wrote:

The only reason I can think of is that the lens stops down more. So Tamron's idea of f4 is a bit smaller than Pentax's. Is it consistent for all apertures?
Click to expand...
Good question I will have to run a test tomorrow. I certainly hope so.
 
Yes, a 50mm lens will show the same field of view where it is a 50mm lens intended for full frame or a 50mm lens intended for APS-C.

Don't worry too much. The focal length may vary depending on how the lens was designed. Neither may be exactly 50mm and that is just the way they were made.

Also, the focal length of the Tamron varies depending upon the distance of the point of focus. Focused close, it is wider than 50mm. At infinity focus, it is 50mm. Or, vice versa... I don't really remember which way it works. [I think this has to do with the lens being 'internal focus'. I learn these things and forget them...]

Whichever way it works, the Tamron may exhibit this effect more or less than the primes.
 
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Thanks everyone. Very helpful.
 
Did you set your exposure manually? I had set it manually when I noticed this difference.

A lens that transmits less light is bad. As you can see, I found I had to set a stop lower shutter.

However, if I had the camera set to an auto mode, I think the meter, looking through the lens, would make the exposures about the same in the end...

I mean, if I had camera in Aperture Priority with the A28 on, I think the camera meter would say 'okay I got this much light so 1/15 second' and if I had the 17-50 on, the camera would think 'okay, less light now so 1/8 second'.

The Tamron costs light but I think the meter will compensate... You just know if you need a faster shutter shooting indoors or something, you might grab your prime. The prime will net you a faster shutter with exposure (histogram) being the same...
 
Tan68 said:
Tan68 wrote:

Did you set your exposure manually? I had set it manually when I noticed this difference.

A lens that transmits less light is bad. As you can see, I found I had to set a stop lower shutter.

However, if I had the camera set to an auto mode, I think the meter, looking through the lens, would make the exposures about the same in the end...

I mean, if I had camera in Aperture Priority with the A28 on, I think the camera meter would say 'okay I got this much light so 1/15 second' and if I had the 17-50 on, the camera would think 'okay, less light now so 1/8 second'.

The Tamron costs light but I think the meter will compensate... You just know if you need a faster shutter shooting indoors or something, you might grab your prime. The prime will net you a faster shutter with exposure (histogram) being the same...
Click to expand...
Seems to me that should be the case but it is not as I had aperture priority set and the ISO and shutter speed was exactly the same for the prime and the Tamron so the meter is not compensating. I am going to play around a bit but I think I will set a positive ev of about 2/3 stop. I am going to check my DA* 60-250 as it will likely need some positive compensation too.
 
I noticed this same issue when I bought a Tamron 17-50mm back in 2010. It exposed darker than my Tamron 28-75mm, as well as several other lenses. I believe that the aperture on the 17-50mm simply does not open up to a full f/2.8, but is closer to f/3.2 or 3.3. I made a lengthy post at the time on the Pentax forum with lots of test pics.

I ended up returning the 17-50mm since I didn't want a lens to expose differently than all my other lenses, and I didn't want less than the f/2.8.
 
Bob Corson said:
Bob Corson wrote:
Bob Corson said:
However, if I had the camera set to an auto mode, I think the meter, looking through the lens, would make the exposures about the same in the end...

I mean, if I had camera in Aperture Priority with the A28 on, I think the camera meter would say 'okay I got this much light so 1/15 second' and if I had the 17-50 on, the camera would think 'okay, less light now so 1/8 second'.

The Tamron costs light but I think the meter will compensate... You just know if you need a faster shutter shooting indoors or something, you might grab your prime. The prime will net you a faster shutter with exposure (histogram) being the same...
Click to expand...
Seems to me that should be the case but it is not as I had aperture priority set and the ISO and shutter speed was exactly the same for the prime and the Tamron so the meter is not compensating. I am going to play around a bit but I think I will set a positive ev of about 2/3 stop. I am going to check my DA* 60-250 as it will likely need some positive compensation too.
Click to expand...
This is why I don't buy the T-Stop argument. Surely if the T-Stop difference is that big it would be like putting an ND filter on, so the camera would meter differently and give a correspondingly different shutter speed or ISO? But in this case it hasn't. It has read the same amount of light and therefore given the same exposure solution.

The test would be if the lenses produced the same or different exposures WIDE OPEN. That way any inaccuracies in how they stop down will be eliminated. If the exposures LOOK the same, then the problem is stop down accuracy, if they look different then it's something else such as T-stops...
 
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OK, that is good for me to know as well. Thanks.
Bob Corson said:
Bob Corson wrote:
Bob Corson said:
Tan68 wrote:
...
Click to expand...
Seems to me that should be the case but it is not as I had aperture priority set and the ISO and shutter speed was exactly the same for the prime and the Tamron so the meter is not compensating. ...
Click to expand...
 
I found the A28 and 17-50 to look, for all my practical purposes, the same wide open.
torrent999 said:
torrent999 wrote:
... The test would be if the lenses produced the same or different exposures WIDE OPEN. That way any inaccuracies in how they stop down will be eliminated. If the exposures LOOK the same, then the problem is stop down accuracy, if they look different then it's something else such as T-stops...
Click to expand...
 
torrent999 said:
torrent999 wrote:
torrent999 said:
Bob Corson wrote:
torrent999 said:
However, if I had the camera set to an auto mode, I think the meter, looking through the lens, would make the exposures about the same in the end...

I mean, if I had camera in Aperture Priority with the A28 on, I think the camera meter would say 'okay I got this much light so 1/15 second' and if I had the 17-50 on, the camera would think 'okay, less light now so 1/8 second'.

The Tamron costs light but I think the meter will compensate... You just know if you need a faster shutter shooting indoors or something, you might grab your prime. The prime will net you a faster shutter with exposure (histogram) being the same...
Click to expand...
Seems to me that should be the case but it is not as I had aperture priority set and the ISO and shutter speed was exactly the same for the prime and the Tamron so the meter is not compensating. I am going to play around a bit but I think I will set a positive ev of about 2/3 stop. I am going to check my DA* 60-250 as it will likely need some positive compensation too.
Click to expand...
This is why I don't buy the T-Stop argument. Surely if the T-Stop difference is that big it would be like putting an ND filter on, so the camera would meter differently and give a correspondingly different shutter speed or ISO? But in this case it hasn't. It has read the same amount of light and therefore given the same exposure solution.

The test would be if the lenses produced the same or different exposures WIDE OPEN. That way any inaccuracies in how they stop down will be eliminated. If the exposures LOOK the same, then the problem is stop down accuracy, if they look different then it's something else such as T-stops...
Click to expand...
There is still a difference wide open. Possibly not quite as much but I am splitting hairs at this point. I have gone through a bunch of photos taken with the Tamron and find that .5 to 1 stop dark depending upon the scene and f stop (to some degree. I have adjusted a bunch of them using the histogram and looks to me like using 2/3+ev should give pretty good results so I am going to try that for a while and see how I like the images I take that way.
 
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