Summary: Although you can probably use (a combination of) photographic neutral density (ND) filters to reduce the same amount of light as a typical solar filter (e.g. Baader solar filter), it is not SAFE for your eyes as it will not filter out the harmful Ultraviolet (UV) and infra-red (IR) light.
The table below shows the reduction in light intensity obtained by using different filters — both photographic ND filters of various Optical Density (OD) and the Baader solar filter. Just from the standpoint of intensity reduction, it is evident that one would require a 12-stop ND filter (OD 3.8) to match the Baader Photographic AstroSolar film. However, the following reasons undoubtedly makes the photographic Neutral Density filters unsuitable for solar photography:
Summary: Estimate the apparent image size of the solar disk and Venus in pixels for certain camera and simulate the ideal observed image.
Disclaimer:In this post, all discussions are based on the apparent image SIZE of the subjects — solar disk and Venus. I have not discussed any issue of resolving fine details, which is very important, and that shall be discussed in another post. Whether you will be able to capture the fine details such as the Granule, Lanes and Bright points on the solar spots, will depend on the ability to resolve fine details called the “spatial resolution”. The spatial resolution that you can achieve depend on the quality of the lens, quality of the sensor, pixel size of the sensor, the focus, presence (or absence) of vibration during the exposure, and lastly but perhaps the most limiting of all in the case of astro-photography is “seeing.” For today’s discussion we are not concerned with any of those, assuming our camera is the perfect camera and there is not air-turbulence.
You may have been wondering whether the focal length of the lens that you have will be “good enough” (see disclaimer) for taking photographs of the transit. The answer to that question really depends on what you want to photograph and the focal-length of the optics that you have at your disposal. While a 300 mm lens attached to a camera with APS-C sensor will suffice (not ideal) for taking pictures of the entire solar disk with Venus (see the simulated images below & some photographs of the Sun that I took during the past few days), you will need focal length beyond 2000 mm if you want to take frame-filling close up Venus in front of a partly visible solar disk.