Macro question

I recently aquired a 28mm prime to reverse mount on the front of my Tamron 90mm, when I was looking through the viewfinder or using the live-view there was no sign of vignetting all looked okay but when I took the picture this was the result




I am thinking it is possibly because the front element of the Tamron is recessed a good distance back in the body, but am still puzzled as to why it is fine in live-view or viewfinder then gives this as the finished result. Any clarification appreciated.

as a side the picture was just so I could work out rough magnification.

It may be that your post processing has made previously minimal vignetting more visible, I've had this on several shots when boosting the contrast. Live-view should be identical to the photo as its a full view of the sensor, unless you had it zoomed in for critical focusing...
Vignetting is easily missed in the viewfinder in my experience, at least if your not specifically looking for it.
FWIW if both lenses are focused at infinity the magnification should be 90/28 (from the focal lengths) or roughly 3x. (The ~8mm view works out much the same)
BTW with most lenses the focusing of the reversed lens will not effect the focus of the image as it only moves the lens mount further from the lens elements - increasing the degree of vignetting.

Nice sharp focus, with a more detailed subject, the centre portion could be quite usable. I suspect a 50mm would stack better though giving you ~2x magnification and probably less vignetting.

FWIW.....


I always "worked" on the reproduction ratio being main lens focal length divided by reversed lens focal length = reproduction ratio.
I.E., 50mm lens reversed on 100mm lens = twice life size.

This is not based on any impirical evidence, nor mathematical expertise...
Did/does this only apply to 35mm, or would it "work" with APSC?

...In fact, I can't remember where I got it from.....!?

But I grag this up from Exa IIb / Pentax MX days.....



Sorry to be so vague.
It's probably my age.


Roger.

(Sorry, can't find a relevant smiley for all this!)

Did you stop the lens down after looking through the viewfinder and composing the shot?
With the aperture wide open any vignetting won't be as obvious.

^^ you had the Tamron at f/10. So, when you looked through it before the shot, the Tamron lens is wide open (no vignetting) but when it took the shot it stopped down to f/10 as instructed (leading to vignetting). Set both wide open and it should be fine; if you have too much light or need more DoF stop down the reversed lens, not the Tamron.

28 onto 90 will give you somewhere around 3.25:1-3.5-ish ( 90/28 ). But it does depend a bit on where you're focused on both, ie actual distance between main and reversed which changes as you set the focus differently. For example if you're at the nearest focus (rather than infinity) on the 28 you have marginally more magnification than were you at infinity

Thanks for the replies, the only processing was the size reduction to 800 px wide should've really mentioned that.

I never thought about which lens to do the stepping on, but good to be told what I have got wrong, now for more practice. I was really surprised at the difference in working distance with the reversed lens, I think given time and practice I am going to like this macro stuff, especially if I can find subjects that are not as flighty, or I speed up my reaction times.