Image stacking and focus rail

Hi folks, I'm after some advice please. I am planning on doing some macro shooting, using focus rails and stacking the images to increase the DoF in the finished image. But I am getting a little confused.

I read about the method in Paul Harcourt Davies' book, 'Digital Close-Up Photography Q&A'. My perception of the method is as follows:

1. Set up camera on rail
2. Focus on nearest focus point, take shot
3. Rack the rail forward by an amount less than the DoF
4. Take shot
5. Repeat previous 3 and 4 until required number of images taken
6. Use software to stack images.


That's OK, but surely if I rack the camera forward, the object will increase in size ever so slightly - just the same as if I stand 20 yards away with a 50mm lens, then walk to 10 yards away, the image will get bigger. So if the images increase in size, even if this is extremely small, won't that affect the stacked image? Causing, at the very least, a blurred edge to the shot? Or am I getting this completely wrong?

All advice and help gratefully received, many thanks in advance.

I use the the focus rail only in order to bring the object within the focus (depth of field) range of the particular lens being used. The range of images to be used for stacking are obtained by varying the point of focus on the object.
Thus the camera is not moved and the image size remains constant.

Roger.

Yes, it would if the stacking software didn't align the images by scaling, rotating and translating them so they match before stacking them. I have used CombineZP occasionally and it certainly does do that. You can also export the aligned images and step through them to check that they are properly aligned.

I *assume* (but do not know) that Photoshop CS6 and other software that provides stacking functionality does something similar.

Cheers
Andrew

Which lens do you use for this? Most change length as you adjust focus, which moves the objective lens in relation to the subject. Doesn't that affect image size?

The only lens I have which doesn't change length as you adjust focus is an old XR Rikenon 300mm f/4.5 APO manual telephoto, which is a bit impractical to use with extension tubes - though I have on a couple of occasions. Even then, CombineZP still did some rescaling before stacking.

Andrew

Gents, many thanks for your replies.

Roger, I had wondered if that was the way to do it, but my problem was that went against what I read in the book, so I had discounted it.

Andrew, that's good news, I hadn't realised the software could compensate for that. On the subject of CombineZP, I've just looked at Alan's site, he only lists up to Vista - do you know if it will work on Win8? And, I'm guessing the answer is no, but will it work with the K-x, with images showing on the screen?

Thanks again.

Focus stacking software compensates for the change in image size as you move the rack/camera/lens forward. It's not a problem.

The procedure you describe is probably the most widely used. Keeping the camera/lens still and changing the focus point will also work, and the image size may still change (because some lenses actually change their focal length slightly when you re-focus at different points in the scene) and the software will still compensate, but . . . you don't have as much fine control using the focus ring on the lens when compared with the minute steps that can be achieved with certain focusing rails or linear stages.

If you have loads of disposable income, have a look at the StackShot.

Thank you Peter.

It might be worth you having a look at this YouTube video: http://www.youtube.com/watch?v=mzVD95-9YOU It's a bit long-winded, but very informative (for me). Have a look at the section starting from 8 mins onward.

Apparently, the best way to shoot is to use a bellows but with the lens fixed and the camera moving to change the point of focus. This means that the "entrance pupil" is in a fixed position (relative to the subject).

Cheers
Andrew

I have used CombineZP on Win7, 8 & 8.1 without any problems. It can be a bit slow on lower performance PCs but on a PC with grunt though I think it is brilliant.

Glyn

Thanks Glyn.

I've not done it myself but know a man who does and he recommends starting at the back and coming forwards.

Hello,

If you have a telecentric lens then this doesn't happen, but not many of us do. Good software will handle the change in size, have a look at zerene stacker. I wouldn't recommend moving bellows extension length during a stack but move (camera + bellows + lens) all on a stage. What you don't want to do though is move the specimen relative to the light because that changes the light during the shoot which isn't great.

Lastly, I have a huge site about all this - http://extreme-macro.co.uk/ - it is zerene stacker stacking with stackshot. Different tools but similar process to how you describe it.

Thanks Nass, I'm now having a good browse round your site!