24-70 vs 24-105 ( 24-70 ) brighter by a stop

Strange but I have been testing this for better than a week taken hundreds of photos. H sees the deal if I use both these lens indoors with a flash an set them the same for example iso 320-640 f 4.0-5.6 and speed 1/60-1/250 no matter what the settings along as they are the same the 24-105 is always one stop darker. Jam amazed at this no explanation.

Odd.
Examples with EXIF please.

One thing immediately stands out though, probably irrelevant, but may be a clue: if you are exposing with flash, the shutter speed shouldn't matter (as long as it is slower than the synch speed of course).

If I'm not mistaken, the "f-stop" value is the ratio of the focal length of the lens to its aperture. So, unless your zoom lenses were set to exactly the same focal length, you will have different results even if you set them to the same f-stop value.

L.Morey wrote in post #11500873
Strange but I have been testing this for better than a week taken hundreds of photos. H sees the deal if I use both these lens indoors with a flash an set them the same for example iso 320-640 f 4.0-5.6 and speed 1/60-1/250 no matter what the settings along as they are the same the 24-105 is always one stop darker. Jam amazed at this no explanation.

This should not be. If the settings are the same (ISO, shutter speed, aperture, AND lighting level), the images should look identical regarding exposure regardless of the lens used.

Shenanigans wrote in post #11500976
If I'm not mistaken, the "f-stop" value is the ratio of the focal length of the lens to its aperture. So, unless your zoom lenses were set to exactly the same focal length, you will have different results even if you set them to the same f-stop value.

This is wrong.

Care to set us straight on the wrong part?

Shenanigans wrote in post #11500976
If I'm not mistaken, the "f-stop" value is the ratio of the focal length of the lens to its aperture. So, unless your zoom lenses were set to exactly the same focal length, you will have different results even if you set them to the same f-stop value.

I am sorry I formgotmto say that I would also set the focal lenth th same up to 70mm
I tried 24mm 35 mm 50mm and 70mm

Hmmm... In that case, my theory is out the window. I'm curious to find out what is going on, please keep us updated and good luck!

I'd love to see some samples pictures with EXIF.

That seems really odd and shouldn't be happening.

samples with exif a must.

My wife formatted the card so in the morning i will reproduce my findings and post them with exif data can't wait to see what all the experts have to say

Shenanigans wrote in post #11501015
Care to set us straight on the wrong part?

What's wrong is that what is commonly, but erroneously referred to as "aperture" is in fact "f ratio". With that constant, the aperture (in its true sense) will change with the focal length.

Xarqi - I didnt know this but it sounds interesting - just so I am clear:
If I keep the f-ratio on the camera at "4.0" for example and I take a shot at 24mm FL then change FL to 105 but keep the f-ratio at 4.0, then for each shot the mm diameter of the aperture will be different?

Makes sense when you look at the simple equation, I just didnt think about it in terms of what actually physically happens.

rusty.jg wrote in post #11501628
Xarqi - I didnt know this but it sounds interesting - just so I am clear:
If I keep the f-ratio on the camera at "4.0" for example and I take a shot at 24mm FL then change FL to 105 but keep the f-ratio at 4.0, then for each shot the mm diameter of the aperture will be different?

Makes sense when you look at the simple equation, I just didnt think about it in terms of what actually physically happens.

That's a good approximation. The diameter involved is not actually of the diaphragm itself though, but of the "entrance pupil", its optical image, which may be different for a compound lens.

For more see:
http://en.wikipedia.or​g/wiki/F-number
http://en.wikipedia.or​g/wiki/Entrance_pupil

Shenanigans wrote in post #11501015
Care to set us straight on the wrong part?

The f-number of a lens is the ratio of the optical aperture to the focal length. Since the f-number already incorporates the focal length into the equation, the same light comes through the zoom lens at all focal lengths.

This is, incidentally, why we mark lenses with the f-number and not the actual aperture.

L.Morey wrote in post #11500873
Strange but I have been testing this for better than a week taken hundreds of photos. H sees the deal if I use both these lens indoors with a flash an set them the same for example iso 320-640 f 4.0-5.6 and speed 1/60-1/250 no matter what the settings along as they are the same the 24-105 is always one stop darker. Jam amazed at this no explanation.

Are you setting the flash for manual exposure? I would not trust E-TTL for this type of testing as the flash power used is not reported in the EXIF.

The biggest difference I've ever seen between lenses in actual brightness is about 1/3 stop. A one stop difference is too hard to believe.