Image shift when shooting macro

Recently I've been playing with focus stacking on some macro photos. I don't own any dedicated macro lenses. I use extension tubes. I find that when I shoot multiple frames focused at different points the image gets bigger/smaller as I change the focus. Not much but enough that the images don't exactly match when doing a focus blend in layers. I'm guessing it is because the front element moves when I change the focus. Do internal focus lenses do the same thing? What about the 'floating focus' lenses?

My internal focus lens does the same to one degree or another.
A simple fix for me is to select all the layers and then Edit->AutoAlign, which almost always fixes that problem.

As any lens is focused closer the magnification increases. Focusing by moving the camera/lens using a focusing rail may solve this problem.

Jim,
I had this same problem & asked this same question on this forum. Someone gave me great tips on it so I'm giving credit to the source. Virtually all lenses will change focal length when you change focus, even the prime lenses. If you stack all the images in Photoshop, each in it's own layer, then use Edit -> AutoAlign in Photoshop to as John suggests, it will align and resize all of the photos in the stack so they line up. It will leave a weird fringing around the edge of the final photo. The trick is to shoot the images with a bit more space around the subject to allow for this cropping. I hope this helps.

Jim and all, in close up photography set the camera on manual after ext tubes, etc., then focus by moving the camera in and out.

Lynn

Thanks everyone. I'm generally happy with the macro images I'm getting using extension tubes but wondered if a dedicated macro lens would be worth the expense when shooting stacked layers to increase DOF. It appears not. FWIW I thought of trying "Auto-Align" but it was dimmed out, perhaps because I opened the RAW files as layers from Lightroom and it still showed the file type as DNG. That and the computer I tried it on is still running CS3.

Jim - It shouldn't matter that you opened the raw files directly. For auto align to work, you also have to select all of the layers you want to align; Otherwise it will be grayed out.

Thanks David. I don't remember if I tried that or not. I know I've done auto align with layered TIFF files so I thought maybe that was the problem. This is the photo BTW. I was testing a new LED ring light that I bought.

Jim, the previous responses are on target about the changing magnification and using auto-align. The changing magnification holds for every lens. Moving the camera avoids changing magnification, but you still get a different view so the image on the sensor changes, not just the focal region. The stacking programs (Helicon Focus or Zerene Stacker, to name two) automatically account for this and keep a fixed image size so you don't have the thin rectangles around the edge that you get with PS and auto-align. As to using extension tubes on a "normal" lens, they work well until you start pushing the magnification beyond around 1/2 life size (1:2). With a true macro that will go to life size magnification (1:1), you can add extension tubes and get close to twice life size (2:1) before optical performance starts to degrade (everything gets soft). So whether or not you need a true macro lens depends on how much magnification you want. DOF depends only on the magnification and f stop, not the lens. BTW, that's a fine stack on the Amaryllis.

Thanks Mark. I know about what affects DOF (been doing this for 50+ years) but have done only a handful of stacked focus images. I'm primarily a landscape photographer who uses a tripod and small f/stops. I've had occasion a couple of times lately though to shoot macro images handheld and the f/stop I could use (and/or the depth of the subject) required multiple frames to capture it all sharp. I have Helicon focus but didn't use it on this image. I decided to blend the layers manually. It was then that I realized that I was getting slightly different sized images (the flower was larger/smaller at different focus points) and I wondered if it was because I wasn't using a dedicated macro lens. From all the replies the answer is "no" and since this is a very small part of my work I've decided not to invest in one. There's other equipment I need worse and I can manage these images with the way I've been working.