[igabc]

I was taking some pictures to compare the Promaster Spectrum 70-300 and the Promaster LD 70-300. I shot using the same exposure/ISO settings but one lens results in brighter pictures. I know that some cameras, maybe also the K3, cheats on ISO when the lens is at full wide aperture, but my tests were made at f10 so... I can't explain the difference.

I also know that some lenses transmit lights better than others but... I think this is too much of a difference.

Both pics were shot at f10 - 1/320 - ISO 400

Does anybody have an explanation?

Attached Images

   

---

Last edited by igabc; 07-04-2015 at 05:06 PM.

[jatrax]

What metering mode are you using?

[alamo5000]

There is a difference between F stops and T stops. F stop is the actual aperture opening. T stop is how much light actually makes it through the lens.

If one lens had say 6 elements in it and the other had like 10... one will be more efficient than the other.

[igabc]

There is a difference between F stops and T stops. F stop is the actual aperture opening. T stop is how much light actually makes it through the lens.

If one lens had say 6 elements in it and the other had like 10... one will be more efficient than the other.

Yes, but I didn't know there could be so much difference.

[Na Horuk]

Many possibilities: 1) One lens or both might have a problem with the aperture blades, so that the chosen value is not what you actually get. You can test this by simply stopping down both lenses to the same aperture value, then look down the barrel to see if they have similar size openings. But beware, since different designs might look like they have different openings - you can only do comparisons like this if the lenses are very similar in construction/design.
2) The weather changed. Even in daylight, the light is not perfectly constant and things can change between shots. A small cloud, some mist, anything. Maybe not in this shot, but its something to keep in mind
3) F and T stops, just like alamo5000 wrote. T is the amount of light that gets transmitted through the lens. Different lenses have different lens designs, different amount of glass, cut in different shapes, and different lens coatings. This means that even though you have the same focal length and the same settings, the photo will not look exactly the same. Differences will be in rendering, distortion, sharpness, colours, contrast, vignetting, chromatic aberrations, and so on.
4) This one is the least likely, but it might be possible the camera is misfiring, that the shutter stays open longer sometimes than other times, or that you had a setting like EV+/-, bracketing, shadow correction enabled in one but not the other shot.
5) Things like filters, lens hood, whether aperture blades are too shiny, whether the lens is damaged, misaligned, and other things that we don't pay attention to or didn't notice can have an effect as well.

I really doubt 2 and 4 are the culprits in this case. 3 seems most likely to me. Of course, it could be a mixture. Now, if the two lenses have the same number of elements/groups and same lens coatings, then you might want to look for other problems. Can you find lens diagrams for the two lenses?

[alamo5000]

Yes, but I didn't know there could be so much difference.

Try to recreate it in a more controlled environment. See what happens and post the results.

Say for example stop down and get a flash... indoors... don't change the settings or location of the flash and see what comes out the other end.

See if it still happens that way.

[igabc]

I actually tried severl times to switch lenses back and forth and I always get the same result. I even tried to shoot at different apertures, same result. So I guess it's a light transmission thing. I never thought the difference could have been this much between 2 lenses.

Ok thanks all!

[dms]

Actually the good thing is you see it at various apertures. That means you can provide the appropriate e.v. adjustment.

[TheOneAndOnlyJH]

I would expect the camera metering should be able to even out any differences. Have you tried the same experiment to see if there is any difference in Av mode with fixed ISO?
Based on your comparison at fixed settings, I would expect two pictures that look about the same but with slightly different shutter speeds when you let the camera meter choose the shutter speed.

If that's the case and all other image quality is equivalent I would say stick to the lens that allows a faster shutter speed since it would give a slight advantage in low light situations.

[ScooterMaxi Jim]

By chance is one of the two an M lens requiring use of the green button? That often yields under-exposure. Given that you see the problem consistently throughout the aperture range, adding compensation should solve it.

[aleonx3]

I suspect that the difference is in the aperture lever mechanism between the two lenses. A sticky/stiff aperture lever may result in darker scene since the camera can not control the aperture properly.

[RKKS08]

A sticky/stiff aperture lever may result in darker scene since the camera can not control the aperture properly.

It is more likely the contrary will happen.

With stop-down metering the lens gets more time to close the aperture for measurement than when actually shooting (so aperture will be wider open than expected).
With ordinary measurement ("A" lenses or newer) measurement is fully open, but closing aperture for shooting will happen too slow, --> over exposure as well.

The real difference of the lenses should be seen if using both of them fully open - but the F-stop printed on the lens is not always correct. See the posting above about T-stop.

[aleonx3]

It is more likely the contrary will happen.

With stop-down metering the lens gets more time to close the aperture for measurement than when actually shooting (so aperture will be wider open than expected).
With ordinary measurement ("A" lenses or newer) measurement is fully open, but closing aperture for shooting will happen too slow, --> over exposure as well.

The real difference of the lenses should be seen if using both of them fully open - but the F-stop printed on the lens is not always correct. See the posting above about T-stop.

You could be right... What I am saying is that it is possible a stiff aperture lever in one of the two lenses may account for the differences (in terms of exposure).