[Jewelltrail]

I have a few questions, but first the background. I am testing the Tamron 90mm Macro f 2.8 with & without the Tamron 1.4xPz-Af MC4 teleconverter.

To setup a fair test comparison, I am striving to equalize the variables. I find it easy to adjust for the %40 increase in focal length with the teleconverter by simply measuring distances and adjusting accordingly. Through this methodology, I can get close to precise images, with respect to size. Also, it is easy to equalize the light since it is dark out and I have a constant supply of incandescent lighting. The subject never moves throughout the test and is parallel to the sensor plane. However, other variables are not so easy to equalize and I am looking for help,

Firstly, I understand the teleconverter subtracts one stop of light, at least in theory. So my first question is, when I make direct comparisons with apertures, what do I compare to what? For example, the shot without the tele @ 2.8--does that get compared to the shot with the tele @ 2.8--which means f4, since a stop of light is lost? Or should I compare a shot @ f4 without the tele to a shot @ f2.8 with the tele?

Secondly, I am looking at the one stop loss in light due to using the tele and am confused. If there is a 1 stop loss in light then a shot @ f2.8 & 1/6th without the tele, should be a closely identical exposure to f 2.8 & 1/3rd with the tele. But when I compare these two exposures, the one with the tele is significantly more overexposed than the one without the tele--leading me to believe there is less than a loss of one stop when using the tele.

I tried comparing exposures, in this manner, at other apertures and got the same results. For example. a shot @ f5.6 & .3 without the tele, should equal a shot @ f5.6 & .6 with the tele. But again. comparing these 2 images the tele one was significantly overexposed. To get comparable exposures @ 5.6, with & without tele, I had to shoot @ .3secs & .5secs, respectively. To demonstrate this complicated (at least to me) phenomenon, I have 2 crops below. The first one was shot without the tele @ 5.6 & .3secs. The second one with the tele was shot @ 5.6 & .5sec. Even more perplexing, the one shot with the tele is a bit more exposed than the one without the tele.

I am looking for all the help I can get with this one--thanks.

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Last edited by Jewelltrail; 09-25-2009 at 09:04 AM.

[Jewelltrail]

I see I stripped EXIF data when I reduced the already cropped shots by another 50 percent to get them on the forum. Here are the original 2 files, shot raw, no PPing--just resampling to get on forum--the exif data is intact here. I present this to verify what I state in opening thread--thanks.

[newarts]

Maybe the inconsistency in exposure has to do with the camera's getting confused about the lens' capabilities. If the camera thinks the lens is in A mode, it will attempt to make aperture adjustments according to information passed to it by the adapter; but this information might be wrong.

I'd try to put everything into manual mode & see what happens. If the camera thinks the lens is in A mode, the f-number will appear in the viewfinder. If so, turn the e-wheel so the f-number shown in the viewfinder is the minimum it will go to; then the camera will not try to stop down the lens at exposure time.

This test should provide a basis from which you can figure out what's going on with the TC adapter & how to resolve the problem.

Good luck & please let us know what you figure out,
Dave

[Jewelltrail]

Thanks Newarts--it is great to get some ideas. Thing is, I only shoot in manual mode. While testing, I was carefully recording all pertinent info as I shot for the test. Things like ISO, File #, shutter speed, & aperture were all selected , on each shot, by me in manual.

Before I hit the remote to get the shot, I would look into the viewfinder and take all data. Once I had focus and all else setup, all I had to do was select each aperture and shutter speed. I have never had an exposure inconsistency before.

Also, if the Tamron 90mm is getting an exposure @ f 5.6 @ .3 of a sec, and the same lens, with tele, @ f 5.6 gets a slighty brighter exposure @ .5 of a sec---I am starting to think the only thing to explain this is the tele does not take one stop away--more like 2/3rds of a stop. I am starting to believe this only because it holds true at aperture settings I setup for the test i.e., 2.8, 4, 5.6, 8.

[troyz]

I see I stripped EXIF data when I reduced the already cropped shots by another 50 percent to get them on the forum. Here are the original 2 files, shot raw, no PPing--just resampling to get on forum--the exif data is intact here. I present this to verify what I state in opening thread--thanks.

What's the subject distance for each image?

Here's what I think is going on: Since the model is the same height in both images, you must have moved the camera farther away and adjusted the focus (effectively, moving the lens closer to the sensor) in the +teleconverter shot. Moving the lens closer to the sensor results in a brighter image.

Suggestion: try repeating the experiment, but don't change the focus adjustment between images. Just install the teleconverter and move the camera to restore focus.

[Jewelltrail]

troyz: What's the subject distance for each image?

Here's what I think is going on: Since the model is the same height in both images, you must have moved the camera farther away and adjusted the focus (effectively, moving the lens closer to the sensor) in the +teleconverter shot. Moving the lens closer to the sensor results in a brighter image.

Suggestion: try repeating the experiment, but don't change the focus adjustment between images. Just install the teleconverter and move the camera to restore focus

Troyz: All variables were kept equal so I could do a fair test. But, yes, since the tele is 1.4x, I shot it %40 further away from the subject than I shot the 90mm without the tele. Of course I had to adjust focus, being further away, which I did manually, with live view 8x.

Shots with 90mm were 50 inches from sensor to subject

Shots with 90mm plus tele were 70 inches from sensor to subject

I have always used this methodology before and it worked great. However, I am new to the tele world and did not realize (or think hard enough about things) to realize the distance between lens and sensor could impact brightness.

It makes sense now. I was trying to get subjects which were precisely equal in size, so I could present a test which was accurate, valid and fair.

I can not thank you enough, because I now understand: something I am always impelled to do in life.

I can now reconstruct a test in precisely the same manner. I will just adjust shutter speed, as best as possible, to achieve close exposures. I really feel better now, since this has been bothering me most of the weekend. Next weekend I will go back to the drawing board, doing it all again.

I must say, the images are awfully good with the 1.4 tele though. Hard to tell them apart from just the 90mm by itself. When I get comparable exposures, I'll present my results here.

THANK YOU!!! This is a great forum!!

[troyz]

Shots with 90mm were 50 inches from sensor to subject

Shots with 90mm plus tele were 70 inches from sensor to subject

Hmmmm. . . I haven't done the math but I don't think that changes the extension enough to give a 1/3 stop change in brightness


Another issue you could run into (if the lens is in "A" mode) is aperture inaccuracy caused by the mechanical linkage in the teleconverter (i.e. maybe the teleconverter moves the aperture lever on the lens a little bit less than it should).

You could control for that by using the aperture ring on the lens instead of setting aperture in the body when you do the +/- teleconverter experiment.

[Jewelltrail]

troyz: Another issue you could run into (if the lens is in "A" mode) is aperture inaccuracy caused by the mechanical linkage in the teleconverter (i.e. maybe the teleconverter moves the aperture lever on the lens a little bit less than it should).

You could control for that by using the aperture ring on the lens instead of setting aperture in the body when you do the +/- teleconverter experiment.

I just thought of something, even if the movement back 20 inches cause you to re-focus, moving sensor closer to lens, the bottom line is you are still getting a brighter image than the non-tele shot, at only 2/3rds smaller aperture. This is still inconsistent ( I think) with the 1 less stop in effect for a 1.4x tele.

So, what you say about contacts would make more sense I think. That would be easy to control--I will use stop down metering next time for sure. That would make more sense for an explanation

[Jewelltrail]

Okay--back to the drawing board. Upon the suggestion made here, I started the test again, but using only manual stopping down of the Tamron 90mm. I quickly encountered problems. My first shot, without the TC, was @ 2.8 and 1/6th of a second. I then manually stopped down to f4, and set the shutter speed to 3/10ths of a sec, or one stop slower. Here is where the problem comes in.

The second shot came out much more overexposed than the first one, even though they should be an equal exposure. In fact, to get similar exposures @ f2.8 & f4, I had to take both shots @ 1/6th of a sec. You would think the f4 would be much more underexposed @ 1/6th, but this is not the case--it is only slightly more underexposed. This is getting to me.

I took the lens off the body and moved the aperture ring. Clearly, the lens stops down to a smaller opening when I move the ring from f2.8 to f4. So what is up? Is it more likely a problem with the lens, or a problem with the body, or just the combination of them both. Below are resampled shots to show my problem--first one is @ f2.8 & 1/6th the second one is @ f4 & 1/6th.

PLEASE HELP---IDEAS?--THANKS