Alafoto Loc: Montgomery, AL

Yes, but if you're shooting at a smaller f/stop (i.e.; f/8, 11, 16) you'll have a really dim finder image to focus with. Ergo, focus wide open, manually stop down to make the exposure, unless in extremely bright light. It would be like trying to focus with the D.O.F preview activated on a MF camera to focus in stop down mode.

Architect1776 Loc: In my mind

You don't use a mirrorless camera?
On the R7 I go to f22 with a bright viewfinder.
Perhaps your camera needs to be repaired.

Alafoto Loc: Montgomery, AL

Perhaps that's the problem.

Shooter41 Loc: Wichita, KS

Dear Architect1776... This morning, I called B & H Photo in New York and was told that NO MANUFACTURER has created a lens converter for my sixty-three-year-old, 1958, Topcon, 300mm, F2.8, 44.7mm screw mount lens. Therefore, I cannot attach my Sony A7R4 mirrorless camera to my ancient manual lens to get to digital photographic files that I can edit on Photoshop. BAD NEWS FOR ME! Thank you for trying to help me. Shooter41

Architect1776 Loc: In my mind

WOW, about the one lens mount no one makes.
Thx for the info.

MDI Mainer

A comprehensive guide to lens mount identification:

https://www.robertallenkautzphoto.com/lens-mount-identification

MDI Mainer

Apparently the Topcor 300 mm, F2.8 was available with a thread-mount to Nikon-mount adapter, and perhaps others, for which a Sony e-mount adapter is available.

Maybe you can find an old adapter or, more likely, an old adapter on a cheap lens. This would likely work, but obviously would not be cost effective. Even damaged this is a pricey lens for what it is.

https://www.ebay.com/itm/194026732605?hash=item2d2ce4fc3d:g:a2AAAOSwsNBgcMKk&amdata=enc%3AAQAIAAAA4KDqCJLA%2BDW4wHYJ57drQUvCLVlcJu1xmlB6VbI0wgXyGxtJUysNGaneZ0qwUW6Z2HcsWGcQNLXQOU72dL%2Bgi8SgFHzLzNeL7vIttcT%2BLuujb%2B9cwTRukbNMt6vhof8erRsPD4zFtQ47smhdJk7eiKaBv09w2lbhKUj7Bm%2Bt2kA5QvssEgJMQQqE1URocJMQQs2DlNh7c7VR09NkLXX8mGkqhgjhJFkO5Uo0fMAqlaGfncLl%2BV75pVv6NbCWyhIYJG9mZdn3gl5ldqTWLI4mjRzUKPrMeOPf6gxSWKHcL%2F4e%7Ctkp%3ABFBMgLehhpdi

amfoto1 Loc: San Jose, Calif. USA

Shooter41...

There is no such thing as a "Topcon 44.7mm screw mount".

Topcon lenses were ONLY ever made with bayonet mounts. They had two versions of external bayonet and one breech mount inverted bayonet.

The diameter of the Topcon BAYONET used on the 300mm f/2.8 was 38mm. In fact, there is not now and never has been a "44.7 screwmount".

The lens REGISTER of Topcon lenses is 44.7mm. "Register" is the measurement from the camera's bayonet mounting flange to the film plane. This is the distance where the lens needs to converge images to bring them into focus. Topcon and Exacta both used a 44.7mm lens register or flange-to-focus distance.

Most likely, your lens has been modified. It wasn't uncommon for that to be done to the Topcon 300mm f/2.8, since for a long time it was the only lens of it's type. Screwmounts are either M39 or M42. Either could easily be confirmed with a ruler or micrometer.

If it is 39mm the most common use of those was Leica rangefinders in the 1950s. A bit less common was a Canon version... same 39mm diameter, but different thread pitch... used on their 1930s and 40s rangefinder cameras. Post WWII Canon began using the same M39 as Leica.

However rangefinder cameras have a shallow register (Leica M = 27.8mm), so there is no way that your lens has been adapted for use upon any rangefinder camera. It simply isn't physically possible. However, there also were some SLRs that used a 39mm screwmount... early Russian Zenit, to be specific. While possible, I think it unlikely what your lens has been adapted to fit. AFAIK, Zenit were the only SLRs to use the M39x1 mount. And even they switched to the M42x1 mount in the 1960s.

M42 is most likely. Or, to be precise, M42x1 (1mm thread pitch) was used on a lot of different SLRs up through the 1960s. It is most commonly called "Pentax screwmount", but actually began with Praktica cameras. It has been so widely used that it is sometimes referred to as the "universal threadmount". Very probably this is what your Topcon lens has been modified to fit, simply because so many different cameras have used it. There are MANY inexpensive M42 to Sony E-mount adapters available. If it were me just guessing without measuring the threads, this is what I would order and try.

It is possible it was instead modified to accept a T-mount (by the way, T stands for "Tamron", who invented it). This is also M42... 42mm in diameter. However it uses a 0.75mm thread pitch, so is not interchangeable with M42x1. T-mount also has a very large (55mm) flange-to-film measurement, to allow adapters for many different systems to be fitted (in comparison, M42x1 uses a 45.6mm flange-to-film dimension). I think this is the 2nd most likely possibility. But it's okay too, because there are also T-mount to Sony E-mount adapters. If Pentax/Praktica M42x1 doesn't fit, try T-mount M42x0.75 (or vice versa).

MDI Mainer

Irrespective of the diameter, it appears that this lens was threaded (like a T-mount) to readily accept different brand camera mounts.

See the comment from the eBay listing cited previously.

I'd be curious as to what camera the OP actually used this lens on when he initially acquired it.

Here is a copy of this lens (also listed on eBay) with what is described as a Topcon mount, which shows the bayonet flanges.

https://www.ebay.com/itm/313260152705?_trkparms=amclksrc%3DITM%26aid%3D1110013%26algo%3DHOMESPLICE.SIMRXI%26ao%3D1%26asc%3D20201210111452%26meid%3D0753edd64d484f13a4f6a287e9b4673c%26pid%3D101196%26rk%3D2%26rkt%3D3%26sd%3D194026732605%26itm%3D313260152705%26pmt%3D1%26noa%3D0%26pg%3D2047675%26algv%3DPromotedRVI&_trksid=p2047675.c101196.m2219&amdata=cksum%3A3132601527050753edd64d484f13a4f6a287e9b4673c%7Cenc%3AAQAIAAAA4BFMeJ9vNoeV5R6diewUBS2pUcn2VmAAiH6K%252B7dWWYaVZCQhiQFQ2VEdnjXg2FUsXG%252FbLHKJj9I8bc9rqCtgNmEvM9tWH8ZYY7dRxfwP69%252BMqmk5342%252FzD%252B6It9byk%252BdlcEvwX4M%252BcHezVt29LRldD2M%252F4psi5xQUnrUirS4fqWbcm90UYubpOrzVeGQtdk3%252B9Yl5Az4%252FNXLSlvFU81pHtoKw9ovoxEa1C7ObpRuJItx0tFqA8r1jjkTr3tHRA1XNedYIoWQS9yKOr2%252FCaPM1SHi5nVv%252FBSIwloDPARG6S%252F%252F%7Campid%3APL_CLK%7Cclp%3A2047675