Possible addapter for AE-1 external lens in front of EOS 4000D

Is there an optional way to use an external lens filter (such as polaroid, or some magnification ones) acquired long ago for use in front of an Canon AE-1, to the front of a recently acquired EOS 4000D. The old one is slightly smaller that the new, and fits nicely, but not securely, within the rim, so obviously the threading will not engage.
This connection would not involve the electronic lens fixtures, which I have to assume would be thoroughly incompatible anyway.
If I'm asking in the wrong place, please recommend where I should look.

Any filter diameter a small amount of lesser diameter than a current lens is 'it depends' upon the FL of the lens...

• if Wide AngleFL, not likely to work;
  
• if telephoto FL, more likely to work.
  

For a more secure fit, you can purchase a step-down ring from well-stocked camera stores and online suppliers. To make sure you have the right one, you'll need to know the filter thread size of both the lens and the filter. These are specified in mm, and they are usually written on the lens and the filter. It might be indicated by a greek letter phi (ϕ).

As Wilt implied, you might experience vignetting, depending on the construction and field of view of your lens, and the size difference between the lens and the filter.

Another factor to consider, since you mentioned a "polaroid": polarizing filters in the Canon AE-1 days were known as "linear" polarizers. These worked fine with manual cameras like the AE-1, but they caused problems with autofocus cameras. I forget whether it was the autofocus or the exposure system that had problems, but you should be aware of this before using this old filter on your 4000D. It won't damage anything, but your images won't look like you expect them to. For modern autofocus cameras, you need what is called a "circular" polarizer.

Thanks. I'll look for the thread sizes info to ask/search for an appropriate step-down ring, per your specs.
As to the possibilities of vignetting and fov, I'm pretty much out of my league here, I suspect, and a kind of 'take what I can get' kind of guy, playing a lot with with Photoshop. I think 'amateur' applies, though I used the AE-1 for a trade pub some 50 years ago.

mcoren wrote in post #19302135
Another factor to consider, since you mentioned a "polaroid": polarizing filters in the Canon AE-1 days were known as "linear" polarizers. These worked fine with manual cameras like the AE-1, but they caused problems with autofocus cameras. I forget whether it was the autofocus or the exposure system that had problems, but you should be aware of this before using this old filter on your 4000D. It won't damage anything, but your images won't look like you expect them to. For modern autofocus cameras, you need what is called a "circular" polarizer.

It's the AF that may be affected. The problem is that a portion of the light from the centre of the frame passes through a semi-silvered section of the reflex mirror, on to the AF Sub mirror, and then down to the floor of the mirror box where the actual AF sensors lie. Linearly polarised light doesn't always play will with the semi-silvered portion of the reflex mirror. At least that is my understanding of the situation

Saying that I have used a linear polariser on an 18-55 USM with a 300D, back in 2005, and I found no issues with getting it to focus correctly. I already had the filter and needed to remove the reflections from some faulty paint surfaces on an almost new Land Rover Defender 90 van. The aluminium body panels were less than a year old, and already suffering from galvanic corrosion from the steel pop rivets they used to "nail" it together. These were photos taken at the long end of the focal length, and very close to MFD, if that might make some difference to the outcome.

I was using Cokin square filter holders back in the day, so a 58mm ring was cheap enough to make use of the filters I had practical. If I already have a linear polariser, well I would try it, being very sure to be keeping a close eye on the AF performance. If it caused issues, then I would make sure that any new polarising filter I acquired was of the circular kind. One other thing to remember is that old polarising filters, especially cheap ones can be pretty low quality optically. Saying that so can chap CP filters too. In the sixteen years that I have been shooting with DSLR's I think the only filters that I have really needed, that can't be done in post, are graduated ND and CPL. Although I do want some straight ND filters for my Sigma 150-600, as I often want to shoot at speeds of less than 1/60s, which with a minimum of ISO 100 can mean needing to shoot at ƒ/22 or smaller, not something that is really recommended. Shame I can't find a 95m two/three stop ND filter for less than £200 that is actually available here in the UK.

Alan