Filter drawer and back focus

[Borderline Bob]

New to AP and a bit confused (what's new), if I put a filter drawer in the train I understand I would need to add (generally) 1mm to the train for back focus purposes when inserting a filter, do I then if not using a filter in the drawer remove the 1mm spacer. However as I'm typing this I do feel a Homer Simpson "d'oh" moment is imminent.

Thanks in advance.

[symmetal]

Back focus spacing is only important if using a field flattener or coma corrector in your imaging train, as an exact distance is required between the flattener/corrector and the image plane for them to work correctly. Adding a filter between the flattener/corrector and the camera will increase this critical distance due to refraction through the filter so an extra mm or so spacing is necessary. You;re right that if the filter is removed then the extra mm spacer would need to be removed.

Getting the spacing right is more critical with larger size sensors. With smaller sensors the spacing distance is not so critical and the change when adding a filter is likely not worth worrying about.

If not using a flattener/corrector them there is no critical back spacing requirement, so as long as you can focus the image then your're fine. Adding a filter will move the focus point back a small amount but your standard focus knob will take care of that for you. 🙂

Alan 

Edited September 22, 2022 by symmetal

[Borderline Bob]

28 minutes ago, symmetal said:

Back focus spacing is only important if using a field flattener or coma corrector in your imaging train, as an exact distance is required between the flattener/corrector and the image plane for them to work correctly. Adding a filter between the flattener/corrector and the camera will increase this critical distance due to refraction through the filter so an extra mm or so spacing is necessary. You;re right that if the filter is removed then the extra mm spacer would need to be removed.

Getting the spacing right is more critical with larger size sensors. With smaller sensors the spacing distance is not so critical and the change when adding a filter is likely not worth worrying about.

If not using a flattener/corrector them there is no critical back spacing requirement, so as long as you can focus the image then your're fine. Adding a filter will move the focus point back a small amount but your standard focus knob will take care of that for you. 🙂

Alan 

Thanks Alan, yep there is a field flattener in the mix.

[symmetal]

As you're using a filter drawer with the flattener, then the easiest way to maintain the spacing is to put a clear glass filter in the filter drawer when you don't want a specific filter. This avoids you having to remove a spacer to compensate. A UV/IR cut filter also called a luminance filter is commonly used as a 'clear' filter as it has the benefit of blocking out of focus IR wavelengths from the sensor.

Alan

[Borderline Bob]

21 minutes ago, symmetal said:

As you're using a filter drawer with the flattener, then the easiest way to maintain the spacing is to put a clear glass filter in the filter drawer when you don't want a specific filter. This avoids you having to remove a spacer to compensate. A UV/IR cut filter also called a luminance filter is commonly used as a 'clear' filter as it has the benefit of blocking out of focus IR wavelengths from the sensor.

Alan

Thanks Alan sounds an ideal solution.