Light Falloff - Super Angulon 90/8 on 6x7 back

Discussion in 'Large Format Cameras and Accessories' started by Wishy, Aug 1, 2011.

  1. Wishy

    Wishy Member

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    Hi All

    Hoping someone might have some practical advice.

    I shoot a fair amount of architecture, and have started using a baby large format camera (Toyo 23G with bag bellows) and 6x7 rollfilm back. (Hopefully I'm welcome here rather than the medium format thread)

    I've got a super angulon 90mm f8 lens (Rather than the 5.6 as I'm told the light falloff is less dramatic)

    I'm starting to get to grips, but i suspect it'll be a time before I start to do much more than shift. All is going well except when I do larger shifts.
    The super anglon clearly has the coverage, but I can see from the screen the image is nowhere near as bright (at f8 anyway). My exposures are way under, despite me thinking "Oh well, give it an extra stop"

    So my question is, does anybody have a decent rule of thumb to calculate the compensation for light falloff in the area I've chosen to "crop" from the image circle?

    Everything else seems to me a case of practice practice practice, although I've stuffed up remarkably few frames so far.
     
  2. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member Moderator

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    The center filter for that lens is in the order of 1.25-1.5 stops and assumes the lens is set at f:22, and still won't correct falloff 100%. I would recommend shooting 6x7 at f:16-22, and if you don't use a center filter, add 1.5 stops for large amounts of front rise, and bracket exposure for color slide film.
     
  3. Wishy

    Wishy Member

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    Yup, going for a constant F22, or bracketing f-stop for the moment if I'm fiddling much with tilts. I'll give 1.5 Stops a go and bracket. (Hey, at least rollfilm is "cheap" to bracket!)

    Thanks for your help
     
  4. domaz

    domaz Member

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    I think you need a wider angle lens if you want to shoot architecture on 6x7. A 90mm is a wide angle for 4x5, on 6x7 it's considered a normal lens. You should think about getting a 47mm Super Angulon or similar 6x9/6x7 wide angle. Dan Fromm has an excellent write-up on 6x9 lenses if you want to get some other ideas for proper wide angle lenses for the format.
     
  5. Wishy

    Wishy Member

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    I've got a 65mm Super Anglon as well, but the shutter is having problems and is off for a service at the moment.
    I'm guessing any compensation i need to do for one will be largely the same as another. I believe I'm right in saying they're the same design just scaled up / down.

    I'm fairly used to the focal lengths I use from my Rollei SL66. I use the 80mm and the 50mm the most for architecture. 90mm seems to be producing some nice stuff for me.

    The 47 might be a bit tight, but I'm keeping my eyes out for one at the right price (No rush, I'm still incompetent, err, inexperienced with the large format camera, and more lenses isn't going to fix that!). Will certainly need a recessed board, and even with that I suspect I might have problems.

    Thanks for the link,will take a look
     
  6. Ian Grant

    Ian Grant Subscriber

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    The fall off on the screen has nothing what so ever to do with the light fall off on the film, it's just the way a ground glass screen works, and the angle you view it at. Even with a 150mm lens on a 5x4 camera I can measure over 3½ stops fall off from center to edge with a plain screen, and that's not reflected in the resulting negatives.

    That doesn't mean theres not slight fall of with a 90mm f8 SA but it's very slight indeed and I've never needed used a center filter, and that's with 5x4 with a 6x7 back there shoulb be no need at all.

    I laso use a 65mm SA a center filter would be imperative for 5x4 colour work but for B&W I've had no issues just dodging the edges, and it's been OK with a 6x9 back.

    Ian
     
  7. Wishy

    Wishy Member

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    Ooh, handy to know.

    My screen is Fresnel as well, I'm guessing that's not going to help. (But it barely needs a towel with an f5.6 lens. I'll sort out some proper black velvet at some point!)
     
  8. Ian Grant

    Ian Grant Subscriber

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    Even with a fresnle there's still some visual fall off, it becomes more noticeable though with Wide angle lenses.

    I made a couple of dark cloths back in 1976 from and old velvet curtain, re-dyed with black Dylon dye, one was stolen from an APUG member's car in Turkey nlast year :D, the others just on it's last legs.

    Ian
     
  9. Wishy

    Wishy Member

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    I'm sure i'll find myself some cheap fabric in Digbeth. Either that or get stabbed. :smile: (Humor applicable to those in the West Midlands area...)
     
  10. Ian Grant

    Ian Grant Subscriber

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    I'm only 20mins from Digbeth :D Found a bag full of fabric, just need the black dye :smile:

    Ian
     
  11. Smudger

    Smudger Member

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    Ian Grant,not uncommonly,has given good advice. I can't imagine what movements you would need to use with an SA 90/8 on 6x7 to get falloff.
     
  12. 2F/2F

    2F/2F Member

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    Ditto. I have used this lens extensively for rail camera movements on 4x5, and never noticed any "weirdness" in my exposures on transparency film. With 6x7, I wouldn't even think to consider it an issue. If your negs are coming out underexposed, I would first look at your metering technique. The number one warning flag would be if you are using a reflected light meter. These meters require constant and diligent attention and compensation to get "ideal" exposures. The simple "point at the scene and use the exposure from the dial" method rarely results in the best possible exposure.
     
  13. Marco B

    Marco B Member

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    X4...

    Agree, checking the metering technique, and ensuring it is really appropriate to the situation and camera setup, seems a first step. I have never had real issues with light fall off with my Super Angulon 75mm on my 4x5, which is a pretty wide angle on that format (equivalent to some 25mm on 35mm film).
     
  14. Wishy

    Wishy Member

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    Reason I asked was the two shots I took with a large shift came out blank (Rather than underexposed). I suspect I did something else wrong*, but as the image was much, much darker I thought it might be something to do with that. I had left my light meter at home that day (Because I'm forgetful, and haven't yet got round to putting together a checklist) so it was sunny 16 and intuition, but the rest came out surprisingly well.
    Were asking for general info as much as anything - I hadn't realised a significantly darker image on the screen didn't translate to a darker image on the film. I'll be shooting mostly slide once I get familiar with the kit - B&W is a lot quicker to process for the moment

    *I'm not expecting something like bellows getting in the way as they're bag bellows, but I'll check carefully next time. I don't think it was the darkslide, but you never know!
     
  15. Ian Grant

    Ian Grant Subscriber

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    Sometimes the dark-slide gets in theway :D it's easy to forget to remove it :smile:

    Ian
     
  16. Ole

    Ole Moderator Staff Member Moderator

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    You have been misinformed. Check Schneider's own illumination curves, and you will see that the falloff is practically identical, when you adjust for the larger image circle of the 5.6 version. That means that at the same distance from the lens axis, the falloff is the same. If anything the f:8 version is very slightly BETTER, within the coverage needed for a 4x5" film size.
     
  17. coa_lund

    coa_lund Member

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    about light falloff

    As pointed out already the apparent light falloff on the ground glass is normally not what one will get on film.

    I think the light falloff is normally (cos v)^4, cosine to the 4th power of the angle of incidence on film. With v=45 deg (corner of a 6x6 neg with a Zeiss Biogon 38 mm) this would be 1/4 or 2 f stops.

    When I see pics taken with Hasselblad SWC published on flickr and other places I usually see a clear light falloff. (I don't have an SWC myself).

    Some fancy modern lenses may partially compensate for this. I happen to have a Zeiss Distagon 40 mm, and with this the light falloff seems to be smaller. (As far as distorsion and shaprpness it is of course outperformed by a Biogon).

    Carl
     
  18. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member Moderator

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    I think this is more a question of your the composition, subject distance (less falloff when focusing on a closer subject), the choice of film, aperture, printing technique and personal taste. If you measure it by photographing a uniform, evenly illuminated surface, you'll see the gradient is there, probably on the order of 2 stops from center to corner on 4x5" with a 75/8 SA, I'd suspect, but with B&W and a subject that looks good with some natural falloff, it's not necessarily an undesirable effect, and it directs attention to the center of the frame.

    The original poster's situation is different, and since I've been shooting a fair amount lately with my 2x3" Technika, I think I can see what the issue is. If you're photographing a tall building, you may be using as much front rise as you can get, putting the whole 6x7 frame close to the edge of the image circle, so you may not see a large gradient within the frame, but there may be a stop or two less exposure than an image with no movements, all other things being equal.

    Of course it may just be an error like forgetting to pull the darkslide, which I'll confess to having done just last week. Note to self: use a less complicated camera when photographing with a four-year-old near the edge of a cliff.