Howtek Question: Auto-Focus?

[sanking]

I recently scanned a few images from a roll of color negative film. The film was exposed in heavy mist so the contrast was very low, and the auto-focus of my Howtek 7500 kept missing the mark. After two misses I recognized the problem, and set the scanner manually to the focus number, 17, that I knew would give the sharpest result.

This got me to thinking. If I am scanning film, the emulsion is always going to be at the same place, right? I mean, a Howtek scanner is a very precise instrument. So if one is scanning film why not just set the focus point and turn off auto-focus entirely? Seems like the focus point will always be right, and auto-focus will always fail with certain kinds of low contrast scene, even though it is bang on for the great majority of negatives and transparencies.

So, those who have a lot of scanning experience, like Lenny, is that how you work?

Sandy

[fishbulb]

I am not Lenny, but what I have read repeatedly is to set the focus manually for your scanner and not to use autofocus. Once you've got the focus set up, you can check it at regular intervals or if you aren't getting the scan quality you're used to. If you are trying to get optimum quality, you may need to refocus manually before each scan, but I don't think that's usually necessary. Using a printed focusing target, instead of a negative or transparency, can make it easier to achieve optimal manual focus. Again, just what I've read, but it seems like reasonable advice.

That said, I still use autofocus on my Howtek 4500, since that is what was recommended to me by the tech who trained me on this scanner. He did, of course, say it might not always work, and said I may have to use manual focus if I couldn't get good results with autofocus. But, I've been getting great results with autofocus enabled, so I haven't felt the need to switch.

[ah693973]

Sandy, I think you use silverfast I use dpl so things might be different for us. With dpl on my 4500 the autofocus occurs on the same spot of the drum and might miss the spot where the film is mounted. I always use a fixed focus when scanning. If it is something I want to do a really nice job on I recheck focus, but 127 usually works for me.

Andy

[Lenny Eiger]

Sandy,

I just had my scanner PMe'd by Evan (that's Preventative Maintenance for anyone unfamiliar with the term). I was picking daughter up from college down there so I drove it down and left it with him for a week. I had been having some problems with the autofocus and it was time for a checkup. All the PMT's are good. However, I apparently as having a little trouble with the beam splitter, that splits the beams into R, G and B. It's essentially a mirror... now replaced. That may have caused some trouble, altho' I don't know exactly how the scanner focuses itself, so I can't say for sure...

I have been testing the autofocus on every scan, by setting it on Auto, then doing it manually to see if I can get a sharper result. I do a detail of the film at .2 by .2 inches, set it to Autofocus (DPL Enabled setting) and watch what it chooses on the panel... Then I set it to Manual and choose a few different settings, detailing ea one... Autofocus is now dialed in... So I am going back to letting Autofocus handle it. At least until I see some grains that aren't sharp...

The scanner is running like a dream. Evan does a great job. He also upgraded me to the latest DPL.

Hope this helps,

Lenny

[Tyler Boley]

Sandy, I'm on Silverfast too. Like you I have found the film edge type is a good place to fix the auto focus spot on soft focus images. As you know you can select the spot for auto focus, rather than the center default, and I usually pick the spot manually someplace where there are good contrasting edges. Like you I presume the precision of the roundness, and consistent diameter, that focus spot should reasonable represent the rest of the film area even if off to the edge.
T

[sanking]

Sandy, I'm on Silverfast too. Like you I have found the film edge type is a good place to fix the auto focus spot on soft focus images. As you know you can select the spot for auto focus, rather than the center default, and I usually pick the spot manually someplace where there are good contrasting edges. Like you I presume the precision of the roundness, and consistent diameter, that focus spot should reasonable represent the rest of the film area even if off to the edge.
T

Thanks for all the replies.

Lenny, glad you have the auto-focus problem on your scanner fixed. That must have been very vexing.

In my case there does not appear to be anything wrong with the scanner, just the anticipated problem of trying to focus on a low contrast negative. Most auto-focus systems in scanners, drums and flatbeds, determine the plane of best focus based on contrast, so low contrast negatives are a big problem.

I am scanning with Silverfast, which gives me the option of selecting either, 1) manual focus, 2) auto-focus with center area, or 3) auto-focus with selected point. I always work with auto-focus with selected point, but in this case nothing in the image area had enough contrast for the focusing mechanism. Tyler makes a great suggestion to choose a spot in the film edge where the film type is visible. If I had done that to begin with I would not have had a problem. In the future that will be my go-to method of focusing.

Sandy

[sanking]

Pictures speak louder than words so here is the image I was trying to scan that would no focus. This is not the final scan, but a low resolution one I made as a contact proof. In trying to focus I selected the trees on top of the mountain slope, though that would give enough contrast but it failed two times. I have actually bumped the contrast up on this .jpeg.

Sandy