Slide holding for Scanning - Ohnar and Other Options

Halina3000 wrote:

Thanks for the comments.

I did some testing with lenses from my old Minolta SLR before buying the secondhand enlarger lens. With a 50mm prime or 28mm plus a x2 teleconverter and the BPM bellows I could not get the whole of the slide onto the sensor. I think I calculated that 70mm would be somewhere near the minimum. My test with my old 80-205 zoom seemed to confirm this but that is a stupidly big lens compared to the bellows. Some articles about digitising slides suggest that enlarger lenses are a better bet so I ended up buying a secondhand 80mm. It's a Rodagon P.

I don't have anything suitable in terms of DSLR lenses BTW.

I would not be against owning something like a macro prime that can do the job but I'm not in the market (i.e. price range) for a new Canon one so I'd probably be looking at something older and manual... perhaps from Nikon that can also use a glassless adapter on the EF mount (MD lenses can't do infinity). I did look for one but nothing compelling presented itself before I bought the enlarger lens.

Resolution and sharpness-wise my results seem very similar to my OpticFilm scanner when zoomed to 1:1. Maybe there is more fine detail that I am not getting out of the slides... probably not in the older 80% because the cameras and lenses were pretty so-so (see my forum handle) but possibly so in my later films when I was getting paid and could afford a Minolta SLR. Trouble is I mostly used print film with the Minolta so there are not very many slides from it.

Getting the entire slide onto the sensor is purely a matter of the relative spacing of slide(subject), lens & sensor. With subject & sensor both two focal distances from the lens you will get 1:1 images - irrespective of what value the focal distance is. 35 mm slides on a FF sensor would fill the frame well at this magnification. If the sensor size is smaller than the slide the relative distance form slide to lens will need to increase & lens sensor distance will need to be adjusted appropriately for focus.

As a quick guide the thin lens formula can be used:

1/focal length = 1/image distance + 1/subject distance

With real lenses the position needed for these measurements are at the lenses entrance & exit principle planes. These are sadly not very obvious (and not always even within the lens dimensions) so some variation around calculated values is needed.