PDA

View Full Version : ISO Rating


Mark T
03-29-2011, 18:31
Dear Roger,

I recently bought some expired Agfa APX 25 film, the first roll of which I developed yesterday. Unfortunately the developed film lacked a lot of contrast and the images appeared quite blurred, as seen below. I knew it was not a problem of imperfect focus, or with my scanning (although that isn't very good). I wasn't sure if the problem was with the film or with my development. I was at the end of my developer and was therefore having to use it twice, compensating with +15% time.

Since I have a couple of your very helpful books, I had a look through your website to see what I could find. Somewhere in there I came across your comments describing the use of expired film, and the potential faults they can have. These faults seemed reflected in my negatives and my problem seemed solved, so thank you very much for that.

I then continued to read you article "Choosing Black and White Films" and that's where I am hoping for some clarification. I have been using HP5+ and DD-X quite a bit recently. In your article you mention that you rate HP5+ at 320 (in camera meter), then develop the film in DD-X, presumably at Ilford's recommended temp/time for a rating of 400 in DD-X? All of which is understandable and okay. However, where I am getting lost, is where you say the film ends up at an equivalent of ISO 650. Does this mean that the negative density has built up beyond that of the ISO for 400 speed film? Does this not mean the film is therefore over-developed/over-exposed? If that is the case, why would you seek that result? I could be way off and have totally missed your point but I can't get my head around what is going on.

Thanks Mate,



Mark

Mcary
03-29-2011, 20:14
Mark

I could be wrong here, but by looking at the example that you posted it appear that you were shooting on an overcast day and that since most of the boats appear to be in focus that you were using a small aperture of say F-8 or so. Most of the time, but not always, the combination of slow film plus small aperture plus overcast day equals slow shutter speeds which in turn can equal blurry images.
Do you happened to remember what aperture and shutter speed that you used for this shot?

Mark T
03-29-2011, 21:05
Hi Mike,

The day was reasonably overcast, but not overly so. There are still some shadows under the boats, for example. I'm pretty conscious of shutter speed and generally shoot at 1/250th or thereabouts and adjust aperture accordingly (as opposed to giving priority to the aperture).

On this occasion though, I was using a slower speed of 1/60th and an aperture of 4 - 5.6.

The thing is, that some of the objects within individual frames are sharpish in places and totally blurred in other places despite being within the same plane of focus. Therefore I am sure the problem was with the film or the development.

Cheers,


Mark

Roger Hicks
03-30-2011, 00:49
Dear Mark,

The (fairly) easy bit first:

Yes, the true ISO of HP5+ in DD-X is about 650. The development time I use is chosen to give me prints on grades 2 and 3 (we always wet print) using our diffuser enlargers. This is probably about 10-15% longer than is optimum for scanning.

Rating the film at 320 may look like a stop over, but it isn't. At least, not always. Broad-area through-lens meters are designed to give optimum results with slide film. Once brightness ranges exceed about 5 stops, this means that you may not get the shadow detail you want.

On a bright, sunny day with deep shadows, EI 320 on a through-lens meter will probably give you the same exposure as EI 650 with a spot meter, using the shadow index and metering the darkest shadow in which you want texture and detail -- if you want shadow detail. On an overcast day, you'll be giving more exposure than you need, but a stop over doesn't really matter.

As the manufacturers always stress, ISO speeds and their development times are starting points, which can and should be modified to give you the results you want.

The more difficult part, warranting a straight 'dunno', is how you can get some things blurred and others not, in the same plane of focus. There are only two obvious candidates, subject movement and camera shake with a focal plane shutter. Because an FP shutter is a travelling slit, it is theoretically possible for camera movement to take place during some parts of the exposure and not other parts. With 35mm this is unlikely, and at 1/60 on a Leica it is effectively impossible. That leaves only subject movement, which also seems deeply unlikely, especially with a static subject.

There is nothing in development that can cause some areas to be softer than others, so I start to suspect the scanner. Try putting a strong magnifier on the negatives to see if the blurring is on the film, or only in the scans.

Cheers,

R.

Mark T
03-30-2011, 01:54
Hi Roger,

Thanks for your very detailed response. Setting the EI according to the type of meter has elucidated how EI 320 can equal EI 650. I guess that on an overcast day therefore, you can set the EI at 400, and on a bright day EI 250 or more - assuming you wanted shadow detail. The corollary of this then is that TTL meters should always be rated at box speed for slide film I suppose?

As to the other point - about the scanning. I did scan another roll immediately after the APX which was new, out of the fridge FP4+ and it was fine. So I really don't think it was the scanner. I probably used the wrong word when I said blurred in some areas and not others. It would be more accurate to say that it was the contrast that varied (making places seem sharpish) across the same plane of focus. This is why I thought the film might have "fogged". I guess all I can do is use another roll of the film with a new lot of developer and if it happens again, I'll know the film is no good. If the film is fine then it'll be my dodgy darkroom skills. The water temperature where I live is currently 28 deg C, so a small time difference can make a big difference.

Thanks again Roger,


Mark

Roger Hicks
03-30-2011, 02:18
Hi Roger,

Thanks for your very detailed response. Setting the EI according to the type of meter has elucidated how EI 320 can equal EI 650. I guess that on an overcast day therefore, you can set the EI at 400, and on a bright day EI 250 or more - assuming you wanted shadow detail. The corollary of this then is that TTL meters should always be rated at box speed for slide film I suppose?

As to the other point - about the scanning. I did scan another roll immediately after the APX which was new, out of the fridge FP4+ and it was fine. So I really don't think it was the scanner. I probably used the wrong word when I said blurred in some areas and not others. It would be more accurate to say that it was the contrast that varied (making places seem sharpish) across the same plane of focus. This is why I thought the film might have "fogged". I guess all I can do is use another roll of the film with a new lot of developer and if it happens again, I'll know the film is no good. If the film is fine then it'll be my dodgy darkroom skills. The water temperature where I live is currently 28 deg C, so a small time difference can make a big difference.

Thanks again Roger,


Mark

Dear Mark,

Yes, subject to equipment variations. It is possible that you might be happier with 1/3 stop more or less, but this would be down to meter inaccuracies and shutter variations, not metering technique. To take an extreme example, the shutter on my 1960s Pentax SV is about a stop slow at commonly used speeds, so I would need to double film speeds when using this camera with slide film.

You're more than welcome to the advice. Further thoughts: don't under-agitate, and make sure the film is fully fixed.

Cheers,

R.

250swb
03-30-2011, 04:44
the images appeared quite blurred, as seen below.
Mark

Have you checked your pressure plate to make sure its not lost its springiness or has been accidentally bent askew?

But depending on the camera there could be other reasons for unexpected blurriness, like the chap who doesn't lock a collapsible lens into place properly etc.

Steve