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Flat Frames

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Please, no giggles. And yes, flat frames are a widely used astrophotography technique. But like many tricks that astrophotographers use, you can make use of flat frames yourself to do some clever things.  If you’re impatient you can skip ahead and discover how Flat Frames can be used below and decide if it’s worth reading the rest of this article.

What Is a Flat Frame?

A flat frame is a normally exposed image with the entire field of view of the image lit as uniformly as possible.  Like dark frames, flat frames are rather dull and uninteresting things to look at. They are visually white or gray and quite boring.

A typical flat frame

Flat Frame: Canon 5D II with 16-35mm f/2.8 L II lens at 20mm

How Do I Create a Flat Frame?

There are many ways to create a flat frame, let me quickly run through a few. First make sure your zoom, aperture and focus are as you will be using them because vignetting and center of field brightness change as you adjust the zoom, aperture or focus. I usually set my camera on aperture priority mode and let the camera meter for me.

But how should I take a flat frame image?

  1. Is your sky cloudless (or uniformly gray)?  If so, point the focused camera at the sky (and only sky) and take a couple of normally exposed images.  Because it’s probably not as uniform as you think, try rotating the camera and pointing it differently to get a good average flat frame. If you are using a very wide-angle lens, it may be hard to get “only sky”.
  2. Take a clean white t-shirt. Drape it over the lens or lens hood. Smooth it out. Shoot a few frames, rotate the shirt, shoot a few more. Obviously if you’re doing this at night, you’ll need a uniform light source – the good news is the color temperature doesn’t matter much.
  3. Select a uniform white or gray display on your iPad, iPhone, Mac or laptop computer. Hold the tablet up against the lens – making sure the lens is completely covered by the display and take several exposures. Rotate the camera or light source to avoid hot spots.

For optimum effectiveness, shoot the flat frames immediately before or after taking your normal shots and do not change the focus, aperture or zoom. Take your flat frames – you should have six to ten of them, and average them. Do not adjust your flat frames. That is, do not brighten, darken or contrast enhance them.

What Can I Do with a Flat Frame?

Did you read this far hoping that flat frames could in fact be useful somehow? Well then, here is the good news. With flat frames you can:

  1. Remove Dust – since dust tends to move around having taken flat frames very near to the same time you took your normal shots increases the effectiveness of the dust removal.
  2. Remove smudges on the sensor
  3. Reduce or eliminate vignetting.

To effectively use a flatframe, however, you must be able to use layers.

Using a flat frame you can get this result

Flat

 

Even though you started with this:

NoFlat

Notice how the bright center of the field has been normalized. You may not think of the center of a lens as being brighter, but you are probably quite familiar with the outer edges being darker, that is, vignetting. Because these images have been cropped the bright area is not centered as you would expect.

What about Dark Frames – Are They Related?

Flat frames and dark frames are not related at all and are used for very different things. Astrophotographers will normally take Dark, Flat, Offset and Light frames… all of which  serve different purposes.  We do recommend taking dark frames for night or low light photography.

How Do I USE a Flat Frame

You’re welcome to look up how to use Flat Frames in any of the references below, but we will be providing a “part two” article with the details.  We’ll also cover Flat Frames both in the next Astrophotography 101 and Photo Manipulation webinars.  If you want to be in the next webinar, please join our subscription list and we’ll let you know when we schedule it!

References:

 

UV Filter for Protection!?

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I see the question asked a lot. Should I get a UV filter to prevent my (expensive) lens from being damaged should something bad happen? Or “the salesperson told me I’d get better photos if I used a UV filter.”

In a nutshell my answers are no and wrong.  The thinking that a $25, $50 or $150 piece of glass in front of a $1,000 lens is going to somehow protect the lens element from harm seems a bit absurd except in a very few scenarios which I’ll address in a moment.  Moreover, to assume that a thousand dollar lens’ image quality will be improved by a filter is unlikely.

Here are some of the arguments for NOT using a filter (clear, UV or any other for that matter).

  1. A filter creates another surface that may cause additional flare, glare or reflection.
  2. For all but the most perfectly polished and coated filters, optical degradation is certain with a filter.
  3. Filters can introduce color casts and vignetting.
  4. Putting a thinner shatterable piece of glass in front of a lens provides a source of sharp shards with which to to scratch the front lens element.
  5. Those who leave a filter on all the time often find their protection becomes unremovable preventing them from using a more useful filter like a polarizer or neutral density filter.

But… That Filter Might Save My Bacon!

Think about it. In what scenario will a filter protect the lens? A blow by a golf ball, baseball or softball? Nah, a direct blow will shatter the filter and drive shards of glass into the front element.  A drop onto the floor, lens first? Maybe. The filter holder may provide a little extra protection to the lens barrel, but again, when the glass filter shatters you’ve got shards of sharp up against your expensive glass.  What about a fall onto a rock?  Yep, a filter might help a little, but a lens hood would help a lot more – as would a lens cap.

Block UV rays

What about the argument that a UV filter will “block UV rays” and improve the contrast and exposure?  That is part true – if you’re shooting film. DSLRs are far less sensitive to UV light than film and that filter is more likely to become a source of glare, flare, internal reflection and vignetting.  That UV filter is also yet another expense and item to carry around.

When Does it Make Sense to use a UV/Clear Filter?

If you have burning metal or corrosive substances flying at your camera, I would certainly prefer that they strike a cheap(ish) piece of replaceable glass rather than my expensive lens. Also, some lenses are only well sealed against rain and dust if you put a filter on them. So an excessively wet, dusty or sandy environment might be a good candidate for filter use.

Under Fire [C_041883]

What Do I Do to Protect My Lens?

Aside from being careful, I would argue that using a lens hood is an almost ideal solution. A lens hood helps keep things away from the front element and it also serves the important additional photographically USEFUL function of keeping off-axis light out of your shot. Off-axis light can cause significant glare and flare and attendant loss of contrast.  Even the best filters are little or no help with off-axis light.

My personal policy is also to “cap the lens” whenever  I am not shooting and definitely before I move anywhere. The cap stays accessible in my back pocket and it goes on the camera before I move it. Much like my seatbelt is always fastened before I start the car.