Charles 46277 Loc: Fulton County, KY

I have read about lens cast when a digital back is used on a view camera with view camera lenses, and I have bought a Phase One P30 back. Using it on a Speed Graphic just to test, magenta is all over the place, and I understand in theory why.

What I don't understand is this--I have used a Canon 650D on the back of a view camera with all sorts of view camera lenses and there is no lens cast--lenses from 135mm to 600mm, most often 360mm. That setup can only use longer lenses (135 or longer), and lens cast is supposed to happen especially with short lenses. But on the p30, I use the same longer lenses and get strong lens cast. For instance, a superb 240mm lens is fine on the 650D but has serious lens cast on the P30. This is by no means wide angle on either sensor size. The P30 sensor is about 4.5 times larger than the APS-C Canon, but 240mm is still a long lens (3x normal) for a cropped medium format camera. (The P30 works perfectly on the H2D Hasselblad.)

I have a better view camera and have not tried it with at--the front and back may have more exact alignment with that.

I have not yet bought the opalescent plate for lens cast correction, but for now I could just use it for black and white pictures. BW looks fine, but lens cast may still be affecting the black and white in a less obvious way (for better or for worse?)

By the way, I thought it would save money to get the cable from P30 to lens shutter with single contact, releasing twice for a picture. Wrong. You have to cock the shutter for the second shot and do it fast (4 seconds), introducing shake. The double cable is far more practical. I tried using the flash contact on the Speed graphic with shutter set to T for the first shot to the P30, then use a regular cable release to the lens for the second click (since the sensor is now exposed at T), so two releases both follow quickly without recocking the lens shutter--but the back was not deceived--it needs a complete release cycle for each trigger release. Clever, but no cigar. A lens with self-cocking shutter could be fired twice without cocking again, and I do have one, but it is too big for the Speed Graphic (Ilex #5 shutter). I may try it on a bigger camera, but it is not portable without a mule team. OK--another question--does the P30 on view camera want M or X sync, or does that matter? I suspect that it wants the shutter all the way open, so that would either be X or a slow M sync. The sync is not (for me) for flash, but the sensor has to be actuated when the lens is open, so that sync is required.

rmalarz Loc: Tempe, Arizona

Perhaps it's indigenous to the Phase One back. https://www.phaseone.com/en/Search/Article.aspx?articleid=2047
--Bob

Charles 46277 Loc: Fulton County, KY

Thanks, Bob--but the question was how come this does not happen on the Canon sensor, but does on separate backs (all of them)? Canon sensor does not require the LCC correction when used on 4x5. Maybe for the P30 I just need to use very very long lenses to avoid this, and then use the LCC correction for shorter lenses. The sensor is much larger.

The Hasselblad software Phocus also provides for such correction (for their backs)--works the same as P30 Capture One (which I don't use).

ORpilot Loc: Prineville, Or

OK, I need to be educated. What is lens cast and LCC correction? I shoot 4x5 but in film.

Charles 46277 Loc: Fulton County, KY

ORpilot, I used to do film on 4x5 amateur (still do sometimes) but I have a medium format digital back now (for Hasselblad) and it will fit on a 4x5 camera instead of the usual sheet film back.

Used in this way, the back picks up color cast across the image, such as blotches of magenta where it does not belong. They say this is because (especially with wide angle lenses) the light from the outer edges of the lens image strike the sensor at an angle rather than straight on, and this picks up coloration from neighboring parts of the sensor.

You can take a picture through a milky white glass with a normal background, and save that image as an example of that lens' color cast pattern for your digital back, then when you take a picture you can use that image (using software for the purpose) to subtract out the offending colors from your photo.

Lot of trouble, yes? But worse--if you use any camera shits, tilts, etc., then you need a color cast correction test shot for each variation (or for the one picture you plan). So for every shot you make one take through the white glass and then the regular picture straight on. Both are used in the software later.

Or you can just convert the digital color image to black and white and forget it.

The strange part is that if I attach the whole digital camera to the back of my 4x5 and shoot with view camera lenses, there is no color cast. That is weird to me.

ORpilot Loc: Prineville, Or

thanks. I understand now. I suppose that it is because a sensor is layered and the actual receptors are behind a lot of layers. Film has no protective layer. Even though color film is 3 or 4 layers and Tri-X is 2 layers. I suppose those layers are much thinner than a sensor. I wonder if Sigma Sensor would be better since it is totally different.

User ID

`

Only one small point that I can address:

Forget M-synch, use X. Many shutters have
X-synch only ... so if the P30's engineering
is based on common sense ?

.

User ID

`

From all your rather thorough descriptions of
combinations of gear and which combinations
bring the magenta or don't, the logical ... tho
strange ... conclusion is that the 4x5 graphic
camera body causes the magenta cast.

While that seems unreasonable, your reports
of what gear combination goes magenta says
you need to consider the unreasonable. You
have nothing to lose, except a magenta cast !

When my back is to the wall and I've run out
of realistic answers, I have ... sometimes ...
been led to a solution by ignoring my doubts
about the "unreasonable" path. Following the
unreasonable path works, when it does work,
cuz it reveals to me that I "didn't know what
it was that I wasn't knowing".

Good luck ;-)

.

Charles 46277 Loc: Fulton County, KY

Yes, makes sense--but if using the Speed Graphic with barrel lens and flash it would be bulbs at M I think.

Charles 46277 Loc: Fulton County, KY

Could perhaps be the camera. But in that case the Canon 650 on the back of that 4x5 would also have the color cast, yes? Unless the small sensor uses only the center of the lens image (no side angle light). When we have better weather I will try different things (a Toyo 4x5, longer lenses).

Charles 46277 Loc: Fulton County, KY

I read somewhere the P30 was a Kodak sensor--and so was Sigma, though both make their own now.

ORpilot Loc: Prineville, Or

Isn’t a Sigma a Fovon which is totally different technology. But i’m Guessing it still has a protective coating.

Charles 46277 Loc: Fulton County, KY

Well, the Speed Graphic does not have X sync. Just bulbs. If the lens on it has x sync, that can be used, but some lenses don't (or have both). I have a supply of flash bulbs for the SG but I am hesitant to use them on digital cameras unless they are attached to the SG as a back. In that case the digital is fired on a long exposure, then the SG is fired on its own focal plane shutter.

The same could be done with the P-30. Fire it with long exposure (manual) then expose via the SG shutter. In that case the flash bulb could be connected to the SG. But the SG shutter cannot be in sync with x--it was designed in 1903 (amazingly with an ingenious 1/1000 shutter speed that is dead accurate). The SG syncs at higher speeds so the flashbulb is lit throughout the exposure, but if the lens sync (not the SG shutter) is used on M, it syncs only at slower speeds and the SG shutter must be locked open.

My Canon 650D and my Hasselblad H2D can be set to fire the x flash either at the beginning or the end of the exposure (same exposure either way), but I am not sure why that matters with x. With M it would matter.

twowindsbear

Post some pix that show your magenta problems. I think you have some sort of light leak where the digital back connects to the camera.

That's my WAG

Good luck