Phase One 150 mm AFD f/2.8 lens cast

[gazwas]

Just wanted to ask anyone with a Phase 150mm if they have issues with lens cast when shot on a full frame back. It's a lovely lens that is very sharp even when fully open across the frame but has this surprising flaw. While I found it unusual for there to be any fall off with such a long lens, I find it even more strange it has LC issues.

See the attached quick example pictures. A picture of my daughter that has a noticeable colour shift at the bottom of the frame and LCC frames uncorrected and corrected to show the difference (please excuse the dust ). These were taken at different times so LCC doesn't match the shot of my daughter but it should give an idea of what I keep seeing.

My response from Phase support was it was normal for my back and the LCC tool does a good job of removing it. Being a tech camera shooter I know all about the LCC tool in C1 but was surprised such a long lens on a DSLR needed correction.

Any thoughts?

 

[Jack]

Was this with an IQ 180 back and were you shooting wide open?

I am of the belief the LCC is a regular fact of life with the IQ180 for accurate color delta, though I have yet to see any color shift using my LS lenses -- which is what I use 90% of the time. Also, wide open apertures create steeper AoI on the sensor, and steeper AoI's are what cause the color shifts. It may be just combinations of certain lens designs with the 180 demand LCC and you've found one in the 150 at wider apertures.

 

[Guy Mancuso]

You are a sick man but love ya anyway. Lol

 

[Guy Mancuso]

So who's the smart one now. Lol I could not resist that one

 

[Jack]

Guy is inferring that the 160 has none of these CC issues the 180 has. Mostly true, but I still had measurable CC issues on the P65+, just not as obviously visible as similar ones on the 180.

 

[Guy Mancuso]

I'm just teasing Jack. Long story. Lol

 

[FredBGG]

What a lovely little baby!

Really small photosites are a problem.

While they are small they are just as deep as lower MP count sensors.

This leads to higher AoI issues.

I have found that there is better performance with Fuji gx680 lenses and Hasselblad V lenses that are designed for harger mirror boxes and as such the rear elements of these lenses are further away from the sensor. Also the much larger rear elements of some of the fuji lenses helps.

 

[gazwas]

I ask here as its with a P65+.

I had heard reports of LC issues even with Phase lenses on the IQ180 but never with the P65. The baby shot was wide open but the LCC capture was at f8 as I suspected the issue may have been the result of the lens being wide open. I first noticed the LC while shooting white ceramics at f16 a couple of days earlier.

As a tech camera only shooter until a week ago when I bought the Phase camera, my quest for an easier workflow when jobs demand seems to have come up against similar issues.

 

[Jack]

If it appears on all frames of your P65+, it may need a new "calibration" file for the back. Call your dealer, they should be able to help you out with that.

 

[David Schneider]

What is meant by "lens cast"?

 

[Jack]

"Lens cast" occurs primarily on technical cameras when the lens it shifted -- the increased AoI (Angle of Incidence) of the light rays can diffract across the sensor grid and generate a striped cast of color shifts across the sensor. In most MF conversion software, there are tools dedicated to removing it; in Capture One for Phase backs, it is called LCC for Lens Cast Correction. Search here and you will find more relevant data on it.

 

[gazwas]

What is meant by "lens cast"?

In simple terms, it's where the light hitting the sensor at a particularly acute angle causes colour shifts in the image. It is usually associated with camera movements on technical cameras.

 

[gazwas]

If it appears on all frames of your P65+, it may need a new "calibration" file for the back. Call your dealer, they should be able to help you out with that.

I can't say I've ever noticed it before or on the Phase 80mm but I will check.

If my back does need adjustment, what exactly is involved with a calibration file and what does it do as I've never heard of this? Is it a visit to Denmark for my back or is it more user friendly than that?

 

[Jack]

A good dealer can do it for you.

 

[gazwas]

Just checked another recent capture of cutlery shot on a grey background that should scream lens cast but this time on a Rodenstock 90mm and they are as clean as a whistle so must rule out the back calibration.

Never simple this digital photography game!

RAW capture straight from the camera below.

 

[Stefan Steib]

Just wanted to ask anyone with a Phase 150mm if they have issues with lens cast ....................

My response from Phase support was it was normal for my back and the LCC tool does a good job of removing it. Being a tech camera shooter I know all about the LCC tool in C1 but was surprised such a long lens on a DSLR needed correction.

Any thoughts?

Gareth
To find out you need to use both lenses with the same lighting. The cutlery shot was with flash equipment I guess or at least sufficient light at around 5000k or up and lowest asa. The Baby shot was on yellowish about 2500k source it seems and probably a longer exposure time.
make a repro setup with 3 different lightings (Flash, Halogen and bulb) and focus to a white cardboard and check different timings.
I´m sure there is a certain lack of linearity which brings out the differing levels of correction of your chips calibration file. This was probably done with 5000k or 6500k and thus does not represent your differing energy levels (electron counts) when you shoot low tungsten stuff.

My guess

Greetings from my new office in Lindenberg /Allgäu
Stefan

 

[gazwas]

GarethI´m sure there is a certain lack of linearity which brings out the differing levels of correction of your chips calibration file.

Thanks for your reply Stefan. I will try as your suggestion with a copy lighting setup

Are you basically saying that my backs calibration file is out of whack or that it's probably tuned for strobe rather than tungsten?

 

[Stefan Steib]

Hi Gareth

The probability is very high that Phase uses a flash setup to calibrate/control the uniformity of the assembled chips. This means at about 6500 k the 3 channels are mostly equalized and are in balance.
If you go down to 2500 k the amount of information transported in the red channel will be more apparent by a factor of 3. Means a calibration that is perfectly suited for 6500 k will show flaws at 2500 k.

This is not a deficiency, it is just a matter of fact that these large chips are not totally uniform, and to make this worse, they do change this with growing temperature ! So run the test with 20 degr. Celsius or 50 degrees Celsius or - 20 degrees Celsius and you also will get differing results.

So to be able to compare lenses on color cast you have to use them in a row, at exact the same lighting, temperature and asa. Otherwise you can forget to say this lens has color cast and this one doesn´t.
What you also may see is differing color from glass transmission shift from lens to lens if you do not white balance. All this adds up and because of this it is extremely difficult to judge "tests" on the subject of color accuracy on digital with differing lenses, focal lenghts and different first apertures.

There is definitely something like lens color cast, but to what extent can be widely discussed (see all reasons beforementioned).

To judge this in a scientific way you would need to have the same setup as Andreas Kraushaar here from the Fogra in Munich who has a lighting setup which can be tweaked for whitepoint and spectral containments inside of the overall whitepoint. He achieves this by using a spectral analyzer from Minolta coupled by a row of differing lightsources who all emit differing spectra and he has dimmers for all of them to fill in parts of the spectra he wants to achieve specific repeatable simulations of typical lightsources.

Just so much for the color accuracy...........

Regards
Stefan

 

[Jack]

Gareth:

Had another thought: The fact that the affected area was at the bottom of you frame could indicate a slight light leak -- especially on longer, hand-held exposures. This would come from the TOP seam of your back<>camera junction. I would tape it over with gaffer and reshoot in similar conditions to see if it's still there.

 

[gazwas]

Thanks Jack/Stefan, I really appreciate your input and I will try your suggestions and report back.

Its one of those things where once you notice it you're always obsessively on the look out.