Reliability. I had a few mucky negs from AG, so I switched to Filmdev. When Filmdev closed, I switched back to AG and the problem hasn't resurfaced.
I also like speed. Not neccessarily "4 hours", but If I post negs out on Monday, I like to have the scans back by Friday. Waiting 2 weeks is "acceptable" and the norm for E-6, but anything over that (pandemic aside) isn't acceptable (to me)
Cost is a factor, but not a massive one. I want to feel like I'm getting value for money, and Filmdev deliver that. I hate scanning, and it's taken me far longer than I thought it would to get acceptable 35mm scans. Both AG and Filmdev deliver exceptional quality [well balanced] "medium" scans that I can print to 13x19 with minimal to no pp. If it's a "not sure if this is any good" roll, the basic scan that comes with a Filmdev job is enough for me to print a decent A4 contact sheet before scanning the winners myself (as Brian says). I have Negative Lab Pro for colour correction and whilst it's still a fiddle to get the colours right, it's good enough for me. No lab offers the service of checking my contact sheet and only scanning the decent shots
That, I'd pay for!
I'll continue to do B&W myself because of the control I have over developing, speed and cost. Nothing would make me move to a lab unless they made selling the chemistry illegal.
I looked at CFL mainly on your recommendation. My issues...
A roll of 35mm with a "medium" scan is £13.50 vs Filmdev's £6. Let's call it twice the price. The CFL scan is bigger @ 3000 x 4100 vs Filmdev 2000 x 3100. In terms of an A3 print (I don't need bigger from 35mm) that's 180ppi vs 120ppi. I'm not a nose-to-the-print kinda guy and my eyesight is terrible, so whilst CFL may say their scans are bigger, and they are, I'm not going to see it. Also return of negs is a pain. You have to wait until they have a few and they bulk send them back to you (at your [unspecified] cost) or you gan get them expedited (again, at [unspecified] cost). Both AG and Filmdev are clear about their postage (Filmdev is free return, AG is free to send)
I really don't get the value-add that makes CFL twice the price. I looked at their "pro" offering, but I am not a photographer who uses templates, so applying broad brush PP is of no benefit to me. Dust spotting is about the only useful service, and either Filmdev/AG use very clean environments, or their software does a grand job. Pro photographers might benefit from their service (which is who I suspect their target market is) but an amateur like me won't.
Finally,
all labs have the really annoying issue of scanning everything at the same resolution. So the benefit of shooting medium format is immediately lost because they're sending you the same sized file as if it were 35mm! I can only assume the software they use scans a preset resolution. CFL actually state on their scan size page that 645 scans are *smaller* than 35mm!!
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Filmdev are more vague but I suspect the process is the same...
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I gave up trying to find sizes on AG's site. They don't seem to want to show it and instead give you the file size... Which is 18Mb for a "medium" scan no matter what sized negative... 35mm or 6x9.
If there was a lab that delivered the same scan resolution across the board (so that bigger negative = bigger scan) I'd very seriously consider moving.
That's my tuppence!