How to do flats with Samyang 135mm F2

[catburglar]

I'm shooting with a Samyang 135mm @ F2 on an APS-C sensor. The raw files have a strong vignette (probably about 2 stops at the extreme corners- but I haven't measured it).

I'm trying to correct with flats when I stack in DSS, but I still end up with a strong vignette in the resulting autosave file.  I've tried using twilight flats with some white paper over the lens to act as a diffuser and a similar technique with a tablet screen. When I review the master flat, it shows the same vignette that the raw images do, so the flat division should work.

I realise I could stop the lens down, but I see many people here using the lens wide open - so there must be a way.

Does anybody have any tip/tricks for getting good flats at F2.

John

[Scooot]

I usually use it at F2.8 or F4 but I haven’t noticed a problem with my flats. I put a piece of white cloth over the lens held by an elastic band. Then hold an image of a white piece of paper I have on my iPad in front of it. I take one shot in av mode then adjust the exposure as I think fit and shoot the flats in tv mode.

[SteveNickolls]

Hi John,

I use my Samyang lens at f/2 with an Astronomik 12nm clip-in filter in the Canon 700D modified DSLR. I have two ways of creating flat frames (either shooting with a manual remote or using BYEOS). For both methods I use a clean, large matt white bethroom tile illuminated evenly and place the lens a 3"- 4" inches away from the tile and not affecting the illumination of the tile. With the Camera in Av setting I then take multiple RAW flat frames (x50) and thereafter use the master flat for subsequent stacking in DSS.  If you get the histogram on the camera/BYEOS between a third and half way across the flats are fine to use. Hope this helps.

Cheers,
Steve

[Fraunhoffer]

Thank you - I was also wondering this very same thing.

 

[happy-kat]

I find AV works a treat but make sure camera in manual focus before turning to AV otherwise focus will invariably change.

[ollypenrice]

I think this is the first time I've heard of under-correcting flats. Over-correction comes up quite often and is often associated with not taking darks-for-flats.

If your exposures were very short I wonder if the flats were being affected by the shutter wipe? You could try slowing them down by dimming the source.

Olly

[Alien 13]

5 minutes ago, ollypenrice said:

I think this is the first time I've heard of under-correcting flats. Over-correction comes up quite often and is often associated with not taking darks-for-flats.

If your exposures were very short I wonder if the flats were being affected by the shutter wipe? You could try slowing them down by dimming the source.

Olly

I had this thought too, if your camera supports it then use mirror lock up too.

Alan

[michael.h.f.wilkinson]

I use a flat panel, but use manual exposure to ensure that the exposure time is long enough not to be affected by the flicker of the panel (which causes banding). I also take dark flats with the same exposure time.

[catburglar]

Thanks for all the tips- exposure time was approz 0.01s - so will try to dim the panel and increase the time, I’ll also take a look at increasing the distance to the panel and might flock the lens hood- wonder if I’m getting some glancing reflections that might be throwing the flat off